#include<bits/stdc++.h>
using namespace std;
int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	int T;
	cin>>T;
	while(T--)
	{
		int n;
		cin>>n;
		n*=2;
		vector<int> x(n+5),y(n+5);
		map<int,vector<int>> sx,sy;
		for(int i=1;i<=n;i++)
		{
			cin>>x[i]>>y[i];
			sx[x[i]].push_back(i);
			sy[y[i]].push_back(i);
		}
		vector<int> vis(n+5);
		vector<pair<int,int>> ans;
		function<int(int,int)> dfs=[&](int u,int ty)//0:x,1:y
		{
			vis[u]=1;
			int rx=0,ry=0;
			while(not sx[x[u]].empty())
			{
				int v=sx[x[u]].back();sx[x[u]].pop_back();
				if(not vis[v])
				{
					int r=dfs(v,0);
					if(r)
					{
						if(rx)ans.emplace_back(r,rx),rx=0;
						else rx=r;
					}
				}
			}
			while(not sy[y[u]].empty())
			{
				int v=sy[y[u]].back();sy[y[u]].pop_back();
				if(not vis[v])
				{
					int r=dfs(v,1);
					if(r)
					{
						if(ry)ans.emplace_back(r,ry),ry=0;
						else ry=r;
					}
				}
			}
			if(not rx and not ry)
			{
				return u;
			}
			else if(rx and ry)
			{
				if(ty)
				{
					ans.emplace_back(u,rx);
					return ry;
				}
				else
				{
					ans.emplace_back(u,ry);
					return rx;
				}
			}
			else
			{
				ans.emplace_back(u,rx|ry);
				return 0;
			}
		};
		vector<int> rem;
		for(int i=1;i<=n;i++)
		{
			if(not vis[i])
			{
				int r=dfs(i,0);
				if(r)rem.push_back(r);
			}
		}
		cout<<ans.size()<<"\n";
		for(unsigned i=0;i<rem.size();i+=2)
			ans.emplace_back(rem[i],rem[i+1]);
		for(auto [u,v]:ans)
		{
			cout<<u<<' '<<v<<"\n";
		}
	}
	
	return 0;
}