#include<cstdio>
#include<iostream>
#include<vector>
#include<algorithm>
#include<utility>
#include<array>
#define RI register int
#define CI const int&
using namespace std;
typedef pair <int,int> pi;
const int N=400005;
int t,n,nx,ny,x[N],y[N],rst[N],vis[N],dep[N],used[N];
vector <pi> v[N],tr[N],ans; vector <int> low[N],rub;
vector <array <int,3>> untr;
inline void addedge(CI x,CI y,CI z)
{
	v[x].push_back(pi(y,z)); v[y].push_back(pi(x,z));
}
inline void DFS1(CI now,CI fa=0)
{
	vis[now]=1; dep[now]=dep[fa]+1;
	for (auto [to,id]:v[now])
	if (to!=fa)
	{
		if (!vis[to]) tr[now].push_back(pi(to,id)),DFS1(to,now);
		else if (!used[id]) used[id]=1,untr.push_back({now,to,id});
	}
}
inline bool DFS2(CI now,CI lst=0)
{
	vector <int> son;
	for (auto [to,id]:tr[now])
	if (DFS2(to,id)) son.push_back(id);
	for (auto x:low[now]) son.push_back(x);
	reverse(son.begin(),son.end());
	if (son.size()%2)
	{
		for (RI i=0;i+1<son.size();i+=2)
		ans.push_back(pi(son[i],son[i+1]));
		if (lst)
		{
			ans.push_back(pi(lst,son.back()));
			return 0;
		} else
		{
			rub.push_back(son.back());
			return 1;
		}
	} else
	{
		for (RI i=0;i+1<son.size();i+=2)
		ans.push_back(pi(son[i],son[i+1]));
		return 1;
	}
}
int main()
{
	//freopen("H.in","r",stdin);
	for (scanf("%d",&t);t;--t)
	{
		RI i; for (scanf("%d",&n),i=1;i<=2*n;++i) scanf("%d%d",&x[i],&y[i]);
		for (i=1;i<=2*n;++i) rst[i]=x[i];
		sort(rst+1,rst+2*n+1); nx=unique(rst+1,rst+2*n+1)-rst-1;
		for (i=1;i<=2*n;++i) x[i]=lower_bound(rst+1,rst+nx+1,x[i])-rst;
		for (i=1;i<=2*n;++i) rst[i]=y[i];
		sort(rst+1,rst+2*n+1); ny=unique(rst+1,rst+2*n+1)-rst-1;
		for (i=1;i<=2*n;++i) y[i]=lower_bound(rst+1,rst+ny+1,y[i])-rst;
		for (i=1;i<=nx+ny;++i) v[i].clear(),tr[i].clear(),low[i].clear(),vis[i]=0;
		for (i=1;i<=2*n;++i) used[i]=0,addedge(x[i],nx+y[i],i);
		ans.clear(); rub.clear();
		for (i=1;i<=nx+ny;++i) if (!vis[i])
		{
			untr.clear(); DFS1(i);
			for (auto [x,y,id]:untr)
			{
				if (dep[x]<dep[y]) swap(x,y);
				low[x].push_back(id);
			}
			DFS2(i);
		}
		printf("%d\n",ans.size());
		for (auto [x,y]:ans) printf("%d %d\n",x,y);
		for (i=0;i+1<rub.size();i+=2) printf("%d %d\n",rub[i],rub[i+1]);
	}
	return 0;
}