#include<bits/stdc++.h>
using namespace std;
constexpr int maxn=4e5+5;
int n;
vector<pair<int,int>> e[maxn];
vector<pair<int,int>> ans;
int dep[maxn],tmp[maxn];
void dfs(int p,int f){
	dep[p]=dep[f]+1;
	int z=0;
	for(auto [x,y]:e[p]){
		if(!dep[x]) dfs(x,p);
		if(dep[x]>dep[p]){
			if(tmp[x]) ans.emplace_back(tmp[x],y),tmp[x]=0;
			else{
				if(z) ans.emplace_back(y,z),z=0;
				else z=y;
			} 
		}
		// cout<<p<<' '<<f<<' '<<x<<' '<<y<<' '<<z<<' '<<ans.size()<<'\n';
	}
	tmp[p]=z;
}
int u[maxn],v[maxn];
inline void solve(){
	cin>>n;n*=2;
	vector<int> a,b;
	for(int i=1;i<=n;i++) cin>>u[i]>>v[i],a.push_back(u[i]),b.push_back(v[i]);
	sort(a.begin(),a.end());a.erase(unique(a.begin(),a.end()),a.end());
	sort(b.begin(),b.end()),b.erase(unique(b.begin(),b.end()),b.end());
	int T=a.size(),N=a.size()+b.size();
	for(int i=1;i<=N;i++) e[i].clear(),tmp[i]=dep[i]=0;
	for(int i=1;i<=n;i++){
		u[i]=lower_bound(a.begin(),a.end(),u[i])-a.begin()+1;
		v[i]=lower_bound(b.begin(),b.end(),v[i])-b.begin()+1+T;
		e[u[i]].emplace_back(v[i],i),e[v[i]].emplace_back(u[i],i);
	}
	for(int i=1;i<=N;i++) if(!dep[i]) dfs(i,0);
	int z=0;
	cout<<ans.size()<<'\n';
	for(int i=1;i<=N;i++) if(tmp[i]){
		if(z) ans.emplace_back(tmp[i],z),z=0;
		else z=tmp[i];
	}
	for(auto [x,y]:ans) cout<<x<<' '<<y<<'\n';
	ans.clear();
}
int main(){
	std::ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
	int t;cin>>t;
	while(t--) solve();
}