#include <bits/stdc++.h> 
#define PII pair<int,int>
using namespace std;
const int MAXN=4e5+5;
int T,N;
int x[MAXN],y[MAXN],xx[MAXN],yy[MAXN],X,Y;
vector<PII>gra[MAXN]; 
int deg[MAXN],cur[MAXN];
bool del[MAXN];
queue<int>q;
vector<int>ans;
int main()
{
	scanf("%d",&T);
	while(T--)
	{
		scanf("%d",&N);
		N<<=1;
		for(int i=1;i<=N;i++)
		{
			scanf("%d %d",&x[i],&y[i]);
			xx[i]=x[i],yy[i]=y[i];
		}
		sort(xx+1,xx+N+1); sort(yy+1,yy+N+1);
		X=unique(xx+1,xx+N+1)-xx-1,Y=unique(yy+1,yy+N+1)-yy-1;
		for(int i=1;i<=N;i++) x[i]=lower_bound(xx+1,xx+X+1,x[i])-xx,y[i]=lower_bound(yy+1,yy+Y+1,y[i])-yy+X;
		for(int i=1;i<=X+Y;i++) deg[i]=cur[i]=0,gra[i].clear();
		for(int i=1;i<=N;i++) del[i]=0;
		for(int i=1;i<=N;i++) 
		{
			deg[x[i]]++,deg[y[i]]++;
			gra[x[i]].push_back({y[i],i});
			gra[y[i]].push_back({x[i],i});
		//	printf("%d-%d\n",x[i],y[i]);
		}
		for(int i=1;i<=X+Y;i++) if(deg[i]&&(deg[i]%2==0)) q.push(i);
ans.clear();
		while(q.size())
		{
			int u=q.front();
			q.pop();
			if(deg[u]&1||!deg[u]) continue;
			int p1=0,p2=0;
			for(int i=cur[u];i<gra[u].size();i++)
			{
				if(p1&&p2) break;
				int v=gra[u][i].first,id=gra[u][i].second;
				cur[u]=i;
				if(del[id]) continue;
				(p1?p2:p1)=v;
				del[id]=1;
				deg[v]--;
				if(deg[v]&&deg[v]%2==0) q.push(v);
				ans.push_back(id);
			}
		//	printf("%d %d %d\n",u,p1,p2);
			deg[u]-=2;
			if(deg[u]!=0) q.push(u);
		}
		printf("%d\n",ans.size()/2);
		for(int i=0;i<ans.size();i+=2)
		{
			printf("%d %d\n",ans[i],ans[i+1]);
		}
		ans.clear();
		for(int i=1;i<=N;i++) if(!del[i]) ans.push_back(i);
		for(int i=0;i<ans.size();i+=2)
		{
			printf("%d %d\n",ans[i],ans[i+1]);
		}
	}
	return 0;
}