#include<bits/stdc++.h>
#define For(i,j,k) for(int i=j;i<=k;++i)
#define IOS {cin.tie(0);cout.tie(0);ios::sync_with_stdio(0);}
using namespace std;
struct node{
    int id,y;
};
vector<node> v[800005],an;
vector<int> s;
map<int,int> mp;
int vx[200005],vy[200005];
int fl[800005],dfn;
int dfs(int x){
    fl[x]=++dfn;
    int las=0;
    for(auto [id,y]:v[x]){
        //cout<<x<<' '<<id<<endl;
        //son
        if (!fl[y]){
            int now=dfs(y);
            if (now) an.push_back(node{id,now});
            else{
                int now=id;
                if (now){
                    if (las) an.push_back(node{las,now}),las=0;
                    else las=now;
                }
            }
        }
        //deeper
        else if (fl[y]>fl[x]){
            int now=id;
            if (now){
                if (las) an.push_back(node{las,now}),las=0;
                else las=now;
            }
        }
    }
    return las;
}
int main(){
    IOS
    int _; cin>>_;
    while(_--){
        int n,x,y,cnt=0; cin>>n; n=n*2;
        mp.clear();
        For(i,1,n){
            cin>>x>>y;
            if (!mp[x]) mp[x]=++cnt;
            if (!mp[y]) mp[y]=++cnt;
            vx[i]=mp[x]; vy[i]=mp[y];
        }
        For(i,1,n){
            x=vx[i]; y=vy[i];
            //cout<<' '<<x<<' '<<y<<endl;
            v[x].push_back(node{i,y+cnt});
            v[y+cnt].push_back(node{i,x});
        }
        cnt*=2;
        dfn=0;
        For(i,1,cnt) if (!fl[i]){
            y=dfs(i);
            if (y) s.push_back(y);
        }
        printf("%d\n",an.size());
        for(auto [x,y]:an) printf("%d %d\n",x,y);
        for(int i=0;i<s.size();i+=2) printf("%d %d\n",s[i],s[i+1]);
        //clear
        an.clear(),s.clear();
        For(i,1,cnt) v[i].clear(),fl[i]=0;
    }
}
/*
3
2
0 0
0 1
1 0
1 1
2
0 0
0 1
0 2
0 3
2
0 0
1 1
2 2
3 3
6
1 2 1 3 1 4
2 1 2 3 2 5
3 1 3 2 3 6
4 1 5 2 6 3
*/