#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,n) for (int i=a;i<n;i++)
#define per(i,a,n) for (int i=n-1;i>=a;i--)
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define SZ(x) ((int)(x).size())
typedef vector<int> VI;
typedef basic_string<int> BI;
typedef long long ll;
typedef pair<int,int> PII;
typedef double db;
mt19937 mrand(random_device{}()); 
const ll mod=1000000007;
int rnd(int x) { return mrand() % x;}
ll powmod(ll a,ll b) {ll res=1;a%=mod; assert(b>=0); for(;b;b>>=1){if(b&1)res=res*a%mod;a=a*a%mod;}return res;}
ll gcd(ll a,ll b) { return b?gcd(b,a%b):a;}
const int N=401000;
vector<PII> e[N];
int vis[N],x[N],y[N],vise[N];
int n;
vector<PII> ans;
VI ext;

void dfs(int x,int pe) {
	vis[x]=1;
	for (auto [y,id]:e[x]) {
		if (id==pe) continue;
		if (!vis[y]) dfs(y,id);
	}
	VI es;
	for (auto [y,id]:e[x]) {
		if (id==pe||vise[id]) continue;
		es.pb(id);
	}
	if (SZ(es)%2!=0) es.pb(pe),pe=-1;
	for (int i=0;i+1<SZ(es);i+=2) {
		if (es[i]!=-1&&es[i+1]!=-1) {
			ans.pb({es[i],es[i+1]});
			vise[es[i]]=1;
			vise[es[i+1]]=1;
		} else {
			ext.pb(es[i]+es[i+1]+1);
		}
	}
	return;
}

void solve() {
	scanf("%d",&n);
	vector<PII> ps;
	ans.clear();
	rep(i,0,2*n) {
		scanf("%d%d",x+i,y+i);
		ps.pb(mp(x[i],0));
		ps.pb(mp(y[i],1));
	}
	sort(all(ps));
	ps.erase(unique(all(ps)),ps.end());
	int m=SZ(ps);
	rep(i,1,m+1) e[i].clear(),vis[i]=0;
	rep(i,0,2*n) {
		int L=lower_bound(all(ps),mp(x[i],0))-ps.begin()+1;
		int R=lower_bound(all(ps),mp(y[i],1))-ps.begin()+1;
		e[L].pb(mp(R,i));
		e[R].pb(mp(L,i));
		//printf("Eg %d %d %d\n",L,R,i);
		vise[i]=0;
	}
	ext.clear();
	rep(i,1,m+1) if (!vis[i]) {
		dfs(i,-1);
	}
	printf("%d\n",SZ(ans));
	assert(SZ(ext)%2==0);
	for (int i=0;i<SZ(ext);i+=2) {
		ans.pb({ext[i],ext[i+1]});
	}
	for (auto [u,v]:ans) printf("%d %d\n",u+1,v+1);
}

int _;
int main() {
	for (scanf("%d",&_);_;_--) {
		solve();
	}
}