#include<bits/stdc++.h>
using namespace std;
constexpr int M=8e5+5;
bool vis[M],flag[M];
int n,num,x[M],y[M],lsh[M],dep[M];
vector<pair<int,int>>ans,adj[M];
int read(){
    int x=0;char ch=getchar();
    while (!isdigit(ch)) ch=getchar();
    while (isdigit(ch)) x=x*10+ch-48,ch=getchar();
    return x;
}
void dfs(int x,int f){
    vis[x]=1;int tmp=0;
    for (auto [y,z]:adj[x]){
        if (z==f) continue;
        if (!vis[y]) dep[y]=dep[x]+1,dfs(y,z);
        if (dep[y]>dep[x]){
            if (flag[z]) continue;
            if (!tmp) {tmp=z;continue;}
            ans.emplace_back(tmp,z);flag[tmp]=flag[z]=1;tmp=0;
        }
    }
    if (tmp&&f) ans.emplace_back(tmp,f),flag[tmp]=flag[f]=1;
}
void solve(){
    n=read()<<1;num=0;ans.clear();
    for (int i=1;i<=n;i++)
        x[i]=read(),y[i]=read(),
        lsh[++num]=x[i],lsh[++num]=y[i];
    sort(lsh+1,lsh+num+1);num=unique(lsh+1,lsh+num+1)-lsh-1;
    for (int i=1;i<=num<<1;i++) adj[i].clear(),vis[i]=0;
    for (int i=1;i<=n;i++) flag[i]=0;
    for (int i=1;i<=n;i++)
        x[i]=lower_bound(lsh+1,lsh+num+1,x[i])-lsh,
        y[i]=lower_bound(lsh+1,lsh+num+1,y[i])-lsh,
        adj[x[i]].emplace_back(y[i]+num,i),
        adj[y[i]+num].emplace_back(x[i],i);
    for (int i=1;i<=num<<1;i++)
        if (!vis[i]) dep[i]=0,dfs(i,0);
    printf("%d\n",(int)ans.size());
    for (int i=1,lst=0;i<=n;i++)
        if (!flag[i]){
            if (!lst) {lst=i;continue;}
            ans.emplace_back(lst,i);lst=0;
        }
    for (auto [x,y]:ans) printf("%d %d\n",x,y);
}
int main(){int T=read();while (T--) solve();return 0;}