#include <cstdio>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <cstring>
#include <array>
#include <algorithm>
#include <queue>
#include <vector>
#include <bitset>
#include <ctime>
#include <cstdlib>
#include <random>
#include <set>
#include <ctime>
#include <map>
#include <stack>
#include <unordered_map>
#define ll long long
#define db double
#define ull unsigned long long
#define reg register
#define fo(a,b,c) for(reg ll a=b;a<=c;++a)
#define re(a,b,c) for(reg ll a=b;a>=c;--a)
#define pii pair<ll,ll>
#define pdd pair<db,db>
#define fi first
#define pb push_back
#define se second
#define ite set<node> ::iterator
using namespace std;
const ll mod=998244353;
inline ll gi()
{
	ll x = 0, f = 1;
	char ch = getchar();
	while(ch < '0' || ch > '9')
	{
		if (ch == '-')
			f = -1;
		ch = getchar();
	}
	while(ch >= '0' && ch <= '9')
	{
		x = (x<<1) + (x<<3) + (ch^48);
		ch = getchar();
	}
	return x * f;
}
ll _=1;
const ll inf=1e17+5;
const ll N=500005;
ll n,cnt=0,vis[N],use[N];
map<ll,ll> mp1,mp2;
vector<pii> g[N],ans;
void dfs(ll u,ll fa)
{
	vis[u]=1;
	for(pii p:g[u])
	{
		ll v=p.fi;
		if(vis[v]) continue;
		dfs(v,u);
	}
	ll las=0,f=0;
	for(pii p:g[u])
	{
		if(use[p.se]) continue;
		ll v=p.fi;
		if(v==fa) f=p.se;
		else
		{
			if(las) ans.pb({las,p.se}),las=0,use[las]=use[p.se]=1;
			else las=p.se;
		}
	}
	if(f&&las)
	{
		use[f]=use[las]=1;
		ans.pb({f,las});
	}
}
void sol()
{
	ans.clear();
	cnt=0;
	mp1.clear();
	mp2.clear();
	n=gi();
	fo(i,1,2*n)
	{
		ll x=gi(),y=gi();
		if(!mp1[x])
		{
			cnt++;
			mp1[x]=cnt;
		}
		if(!mp2[y])
		{
			cnt++;
			mp2[y]=cnt;
		}
		x=mp1[x],y=mp2[y];
		g[x].pb({y,i});
		g[y].pb({x,i});
	}
	fo(i,1,cnt)
	{
		if(vis[i]==0)
		{
			dfs(i,0);
		}
	}
	cout<<ans.size()<<'\n';
	for(pii p:ans)
	{
		cout<<p.fi<<" "<<p.se<<'\n';
	}
	vector<ll> vec;
	fo(i,1,2*n)
	{
		if(use[i]==0) vec.pb(i);
	}
	ll sz=vec.size();
	for(ll i=0;i<sz;i+=2)
	{
		cout<<vec[i]<<" "<<vec[i+1]<<'\n';
	}
	fo(i,0,cnt)
	{
		vis[i]=0;
		g[i].clear();
	}
	fo(i,0,2*n) use[i]=0;
}
int main()
{
	_=gi();
	while(_--)
	{
		sol();
//		printf("\n");
	}
	return 0;
}