// {{{ y0105w49 template 24K21
// hi mom
#ifndef NULL
#ifdef __GNUC__
#ifndef __clang__
// #include <bits/stdc++.h>
#include <bits/extc++.h>
#include <tr2/dynamic_bitset>
#define EXTS
#else
#ifdef ARST
#include <bits/clang++.h>
#else
#ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <cwchar>
#include <cwctype>
#if __cplusplus >= 201103L
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
// #include <cstdalign>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
// #include <cuchar>
#endif
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#if __cplusplus >= 201103L
#include <array>
#include <atomic>
#include <chrono>
#include <codecvt>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#endif
#if __cplusplus >= 201402L
#include <shared_mutex>
#endif
#if __cplusplus >= 201703L
#include <any>
#include <charconv>
// #include <execution>
#include <filesystem>
#include <optional>
// #include <memory_resource>
#include <string_view>
#include <variant>
#endif
#if __cplusplus >= 202002L
#include <barrier>
#include <bit>
#include <compare>
#include <concepts>
#if __cpp_impl_coroutine
# include <coroutine>
#endif
#include <latch>
#include <numbers>
#include <ranges>
#include <span>
// #include <stop_token>
#include <semaphore>
// #include <source_location>
// #include <syncstream>
#include <version>
#endif
#if __cplusplus > 202002L
// #include <expected>
// #include <spanstream>
#if __has_include(<stacktrace>)
# include <stacktrace>
#endif
#include <stdatomic.h>
#endif
#endif
#endif
#else
#error "unsupported"
#endif
#endif
using namespace std;
#ifdef ARST
#define JO 1
#define OJ 0
#else
#define JO 0
#define OJ 1
#endif
#define STR(x) #x
#define GCCDIAG(s) _Pragma(STR(GCC diagnostic s)) static_assert(true)
#define Wsave GCCDIAG(push)
#define Wpop GCCDIAG(pop)
#define Wsupp(w) GCCDIAG(ignored "-W" w)
#define Wpush(w) Wsave; Wsupp(w)
#define typeof __typeof__
namespace gbd_ns {
	template<typename C> struct is_iterable {
		template<class T> static long check(...);
		template<class T> static char check(int,typename T::const_iterator = C().end());
		enum {
			value = sizeof(check<C>(0)) == sizeof(char),
			neg_value = sizeof(check<C>(0)) != sizeof(char)
		};
	};
	template<class T> struct _gbd3C;
	template<class T> ostream &_gbd3(ostream &os,const T &x) { return _gbd3C<T>::call(os,x); }
	template<> ostream &_gbd3(ostream &os,const string &x) { return os<<'"'<<x<<'"'; }
	template<> ostream &_gbd3(ostream &os,char *const &x) { return os<<'"'<<x<<'"'; }
	template<class T> ostream &_gbd3_5(ostream &os,const T &x) { return _gbd3(os,x); }
	template<class A,class B> ostream &_gbd4(ostream &os,const pair<A,B> &p) {
		_gbd3(os<<'(',p.first);
		_gbd3(os<<',',p.second);
		return os<<')';
	}
	template<class T,size_t N> struct _gbd4_tupleC {
		static void call(ostream &os,const T &t) {
			_gbd4_tupleC<T,N-1>::call(os,t);
			os<<','<<get<N-1>(t);
		}
	};
	template<class T> struct _gbd4_tupleC<T,1> {
		static void call(ostream &os,const T &t) { os<<get<0>(t); }
	};
	template<typename... Types> ostream &_gbd4(ostream &os,const tuple<Types...> &t) {
		os<<'(';
		_gbd4_tupleC<tuple<Types...>,sizeof...(Types)>::call(os,t);
		return os<<')';
	}
	template<> ostream &_gbd4(ostream &os,const tuple<> &t) { (void)t; return os<<"()"; }
	template<class T> ostream &_gbd4(ostream &os,const T &x) {
		return os<<x;
	}
	template<class T> struct _gbd3C {
		template<class U=T> static ostream &call(ostream &os,enable_if_t<is_iterable<U>::value,const T> &V) {
			os<<"{";
			bool ff=0;
			for(const auto &E:V) _gbd3_5<decltype(E)>(ff?os<<",":os,E), ff=1;
			return os<<"}";
		}
		template<class U=T> static ostream &call(ostream &os,enable_if_t<is_iterable<U>::neg_value,const T> &x) {
			return _gbd4(os,x);
		}
	};
	template<class T,typename... Args> ostream &_gbd2(ostream &os,bool,vector<string>::iterator nm,const T &x,Args&&... args);
	ostream &_gbd2(ostream &os,bool,vector<string>::iterator) { return os; }
	template<typename... Args> ostream &_gbd2(ostream &os,bool fi,vector<string>::iterator nm,const char *x,Args&&... args) {
		return _gbd2(os<<(fi?"":"  ")<<x,0,nm+1,args...);
	}
	template<class T,typename... Args> ostream &_gbd2(ostream &os,bool fi,vector<string>::iterator nm,const T &x,Args&&... args) {
		return _gbd2(_gbd3<T>(os<<(fi?"":"  ")<<*nm<<"=",x),0,nm+1,args...);
	}
	vector<string> split(string s) {
		vector<string> Z;
		string z="";
		s+=',';
		int dep=0;
		for(char c:s) {
			if(c==',' && !dep) Z.push_back(z),z="";
			else z+=c;
			if(c=='(' || c=='{' || c=='[') ++dep;
			if(c==')' || c=='}' || c==']') --dep;
		}
		return Z;
	}
	template<typename... Args> ostream &_gbd1(ostream &os,const string &nm,Args&&... args) {
		return _gbd2(os,1,split(nm).begin(),args...);
	}
	template<typename... Args> string _gbd1(const string &nm,Args&&... args) {
		ostringstream oss;
		_gbd2(oss,1,split(nm).begin(),args...);
		return oss.str();
	}
}
bool DBG=1,EMACS=0;
#define dbg(...) (JO&&DBG?gbd_ns::_gbd1(cerr<<"\033[38;5;5m"<<__FILE__<<":"<<__LINE__<<(EMACS?":note: ":": "),#__VA_ARGS__,__VA_ARGS__)<<"\033[0m"<<endl:cerr)
#define dbgt(...) dbg(fmt_time(),__VA_ARGS__)
#define fmt(...) gbd_ns::_gbd1(#__VA_ARGS__,__VA_ARGS__)
template<class Fun> struct _y_combinator_result {
	Fun _fun;
	template<class T> explicit _y_combinator_result(T &&fun) : _fun(forward<T>(fun)) {}
	template<typename... Args> decltype(auto) operator()(Args&&... args) {
		return _fun(ref(*this),forward<Args>(args)...);
	}
};
template<class Fun> [[nodiscard]] decltype(auto) fix(Fun &&fun) {
	return _y_combinator_result<decay_t<Fun>>(forward<Fun>(fun));
}
#define nop void()
#define sz(x) (int((x).size()))
#define all(v) (v).begin(),(v).end()
#define forenum(i,...) for(int i:{-1}) for(__VA_ARGS__) if(++i,0) assert(0); else
#define forenumll(i,...) for(long long i:{-1}) for(__VA_ARGS__) if(++i,0) assert(0); else
#define forbs(k,i,bs) for(ptrdiff_t k=0,i=(bs)._Find_first();i<(ptrdiff_t)(bs).size();i=(bs)._Find_next(i),++k)
#define fordbs(k,i,bs) for(ptrdiff_t k=0,i=(bs).find_first();i<(ptrdiff_t)(bs).size();i=(bs).find_next(i),++k)
#define get(x,i) get<i>(x)
template<class T> T &smin(T &x,const T &y) { return y<x?x=y:x; }
template<class T> T &smax(T &x,const T &y) { return y>x?x=y:x; }
template<class T> bool inb(const T &x,const T &l,const T &r) { return l<=x&&x<=r; }
template<class T> bool cinb(const T &x,const T &l,const T &r) { return l<=r?l<=x&&x<=r:l<=x||x<=r; }
#define fi first
#define se second
#define pb push_back
#define eb emplace_back
template<template<class> class C,class T> T popv(C<T> &v) { T z=v.back(); v.pop_back(); return z; }
template<template<class> class C,class T> T popq(C<T> &v) { T z=v.top(); v.pop(); return z; }
template<template<class> class C,class T> T pops(C<T> &v) { T z=*v.begin(); v.erase(v.begin()); return z; }
template<template<class,class> class C,class K,class V> pair<K,V> popm(C<K,V> &v) { pair<K,V> z=*v.begin(); v.erase(v.begin()); return z; }
template<template<class> class C,class T> void erase1(C<T> &v,const T &x) { v.erase(v.find(x)); }
template<template<class> class C,class T> int lbi(C<T> &v,const T &x) { return int(lower_bound(all(v),x)-v.begin()); }
template<template<class> class C,class T> int findi(C<T> &v,const T &x) { auto it=lower_bound(all(v),x); return it!=v.end()&&*it==x?int(it-v.begin()):-1; }
template<class V> int sortu(V &v) { sort(all(v)); int z=int(unique(all(v))-v.begin()); v.resize(z); return z; }
template<typename T,typename... Args> T tee(T (*f)(Args... args),Args&&... args) { T z=f(forward<Args>(args)...); cout<<z<<endl; return z; }
template<typename... Args> void tee(void (*f)(Args... args),Args&&... args) { f(forward<Args>(args)...); }
#ifdef EXTS
template<class S,class T> using omap=__gnu_pbds::tree<S,T,less<S>,__gnu_pbds::rb_tree_tag,__gnu_pbds::tree_order_statistics_node_update>;
template<class T> using oset=omap<T,__gnu_pbds::null_type>;
template<class T> using rope=__gnu_cxx::rope<T>;
using dbitset=tr2::dynamic_bitset<>;
#endif
constexpr int e0=1, e1=10, e2=100, e3=1000;
constexpr int e4=10*e3, e5=100*e3, e6=1000*e3;
constexpr int e7=10*e6, e8=100*e6, e9=1000*e6;
constexpr long long e10=10LL*e9, e11=100LL*e9, e12=1000LL*e9;
constexpr long long e13=10*e12, e14=100*e12, e15=1000*e12;
constexpr long long e16=10*e15, e17=100*e15, e18=1000*e15;
constexpr __int128_t e21=__int128_t(e3)*e18, e24=__int128_t(e6)*e18, e27=__int128_t(e9)*e18;
constexpr __int128_t e30=e3*e27, e33=e6*e27, e36=e9*e27;
using ulll=__uint128_t;
using lll=__int128_t;
using ull=unsigned long long;
using ll=long long;
using uint=unsigned int;
using ushort=unsigned short;
using uchar=char;
using ld=long double;
#ifdef EXTS
using lld=__float128;
#endif
long long START_TIME=chrono::duration_cast<chrono::microseconds>(chrono::steady_clock::now().time_since_epoch()).count();
inline long long now_U_03BC_s() { return chrono::duration_cast<chrono::microseconds>(chrono::steady_clock::now().time_since_epoch()).count()-START_TIME; }
const char *fmt_time(long long U_03BC_s=now_U_03BC_s()) { static char dur[20]; sprintf(dur,"%llu.%03llus",U_03BC_s/e6,(U_03BC_s%e6)/e3); return dur; }
#define timed(cb) do { dbg("timed "#cb" ..."); unsigned long long start=now_U_03BC_s(); cb; dbg("timed "#cb" took",fmt_time(now_U_03BC_s()-start)); } while(0)
int arg1; bool inp; vector<string> args;
unsigned seed=unsigned(JO&&getenv("sd")?atoi(getenv("sd")):OJ?START_TIME:START_TIME%e5);
mt19937 igen(seed<<1),gen(seed<<1|1);
#define irand(...) (assert(!inp),_rand(igen,__VA_ARGS__))
#define rand(...) _rand(gen,__VA_ARGS__)
template<class T> enable_if_t<numeric_limits<T>::is_integer,T> _rand(mt19937 &g,T l,T r) { return uniform_int_distribution<T>(l,r)(g); }
template<class T> enable_if_t<numeric_limits<T>::is_integer,T> _rand(mt19937 &g,T n) { return _rand(g,T(1),n); }
[[deprecated]] int _rand(mt19937 &g) { return _rand(g,0,numeric_limits<int>::max()); }
template<class T> enable_if_t<numeric_limits<T>::is_iec559,T> _rand(mt19937 &g,T l,T r) { return uniform_real_distribution<T>(l,r)(g); }
bool _rand(mt19937 &g,double p) { return bernoulli_distribution(p)(g); }
template<class T> T _rand(mt19937 &g,initializer_list<T> il) { return *(il.begin()+_rand(g,0,(int)il.size()-1)); }
template<class T> T _rand(mt19937 &g,double p,T a,T b) { return _rand(g,p)?a:b; }
template<class T> T _rand(mt19937 &g,initializer_list<T> il,initializer_list<double> wt) { assert(il.size()==wt.size()); return *(il.begin()+discrete_distribution<int>(wt)(g)); }
#define random_shuffle(...) static_assert(false,"random_shuffle deprecated, use shuffle")
#define ine(x,e) (inp?cin>>(x),nop:((x)=(e),nop))
#define inr(x,...) ine(x,irand(__VA_ARGS__))
#define endl '\n'
string garb;
void exit0() { DBG=1; dbgt("gg (early)",seed); exit(0); }
#ifndef MAIN
#define MAIN _main
#endif
void MAIN();
int32_t main([[maybe_unused]]int argc,[[maybe_unused]]char *argv[]) {
	ios_base::sync_with_stdio(0); cin.tie(0); cin.exceptions(ios_base::failbit | ios_base::badbit);
	arg1=0,args={argv,argv+argc};
	if(sz(args)>1) {
		if(args[1][0]=='i') freopen((string(__FILE__).substr(0,string(__FILE__).find('.'))+"."+args[1].substr(1)+".in").c_str(),"r",stdin);
		else if(args[1][0]=='I') freopen(args[1].substr(1).c_str(),"r",stdin);
		else arg1=stoi(args[1]);
	}
	inp=!arg1;
	if(JO && getenv("EMACS")) EMACS=1;
	dbgt(arg1,seed,args);
	#ifdef QUIET
	DBG=0;
	#endif
	MAIN();
	DBG=1;
	dbgt("gg;wp",seed);
	return 0;
}
constexpr int inf=e9+99;
constexpr ll linf=1LL*e9*e9+99;
#if __cplusplus >= 202002L
constexpr long double U_03C4__ld=2*numbers::pi_v<long double>;
#else
const long double U_03C4__ld=2*acosl(-1);
#endif
#define U_03C4_ ((flt)U_03C4__ld)
constexpr long double U_03B5__ld=1e-8l;
#define U_03B5_ ((flt)U_03B5__ld)
// }}}
using flt=double; //CARE
constexpr int P=e9+7;//998'244'353;





auto solve() { /* CURSOR START */
	int n; ine(n,arg1);
	priority_queue<pair<ll,int>> pq;
	for(int i=1;i<=n;i++) {
		ll x; inr(x,e18); pq.push({x,i});
	}
	vector<pair<ll,int>> ans;
	for(;sz(pq);) {
		auto [x,i]=popq(pq);
		auto [y,j]=popq(pq);
		assert(bit_width((ull)x)==bit_width((ull)y));
		if(sz(pq) && bit_width((ull)pq.top().fi)==bit_width((ull)x)) return cout<<-1<<endl,nop;
		ans.pb({x,i});
		// dbg(sz(pq),pq.top(),x,i,y,j,x^y,j);
		if(x!=y) pq.push({x^y,j});
	}
	cout<<sz(ans)<<endl;
	for(auto [x,i]:ans) cout<<i<<' '<<x<<endl;
}



void _main() { int NTC=1;
	ine(NTC,5);
	for(int TC=1;TC<=NTC;TC++) {
		// cout<<"Case #"<<TC<<": ";
		tee(solve);
	}
	if(JO&&inp) assert((cin>>ws).eof());
}