#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
#define rep(i, l, r) for (int i = l; i <= r; ++i)
const int N = 4e5 + 5;
struct edge { int v, id; } ; 
int n, T, cx, cy, ans, num, lst, x[N], y[N], p[N], dx[N], dy[N], vis[N], dep[N]; 
vector <edge> G[N]; 

void add (int u, int v, int id) {
  G[u].push_back((edge){v, id});
  G[v].push_back((edge){u, id}); 
}
void dfs (int u, int f, int ef) {
  dep[u] = dep[f] + 1, vis[u] = 1;
  num += G[u].size(); 
  for (auto e : G[u]) if(!vis[e.v]) dfs(e.v, u, e.id); 
  lst = 0; 
  for (auto e : G[u]) if(dep[e.v] > dep[u] && !p[e.id]) {
    if(lst) p[lst] = e.id, p[e.id] = lst, lst = 0; 
    else lst = e.id; 
  }
  if(lst && f) p[ef] = lst, p[lst] = ef; 
}

int main () {
  ios :: sync_with_stdio(0), cin.tie(0), cout.tie(0); 
  cin >> T;
  while (T--) {
    cin >> n, cx = cy = ans = 0; 
    rep(i, 1, n << 1) {
      cin >> x[i] >> y[i], p[i] = 0; 
      dx[++cx] = x[i], dy[++cy] = y[i]; 
    }
    sort(dx + 1, dx + cx + 1); 
    sort(dy + 1, dy + cy + 1); 
    cx = unique(dx + 1, dx + cx + 1) - dx - 1; 
    cy = unique(dy + 1, dy + cy + 1) - dy - 1; 
    rep(i, 1, cx + cy) vis[i] = 0, G[i].clear(); 
    rep(i, 1, n << 1) {
      x[i] = lower_bound(dx + 1, dx + cx + 1, x[i]) - dx; 
      y[i] = lower_bound(dy + 1, dy + cy + 1, y[i]) - dy; 
      add(x[i], y[i] + cx, i); 
    }
    rep(i, 1, cx + cy) if(!vis[i]) num = 0, dfs(i, 0, 0), ans += num / 4; 
    cout << ans << '\n'; 
    lst = 0; 
    rep(i, 1, n << 1) {
      if(!p[i]) {
        if(lst) cout << lst << ' ' << i << '\n', lst = 0; 
        else lst = i; 
      } else if(i < p[i]) cout << i << ' ' << p[i] << '\n'; 
    }
  }
  return 0; 
}