#include <bits/stdc++.h>
#define L 400005
using namespace std;

int t;
int n, nodeCnt;
pair <int, int> in[L];
vector <pair <int, int>> G[L];
map <int, int> linNorm, colNorm;
vector <int> ans[L];
int lev[L];
bool vf[L];

void init() {
  for (int i = 1; i <= nodeCnt; i++) {
    G[i].clear();
    ans[i].clear();
    lev[i] = 0;
  }
  for (int i = 1; i <= n; i++)
    vf[i] = false;
  linNorm.clear();
  colNorm.clear();
}

void DFS(int node){
  for (auto it : G[node]){
    if (!lev[it.first]){
      lev[it.first] = lev[node] + 1;
      DFS(it.first);
    }
    if (lev[it.first] > lev[node]){
      if (ans[it.first].size() % 2)
        ans[it.first].push_back(it.second);
      else
        ans[node].push_back(it.second);
    }
  }
}

void solve() {
  init();
  cin >> n;
  n *= 2;
  int cnt = 1;
  for (int i = 1; i <= n; i++) {
    cin >> in[i].first >> in[i].second;
    if (!linNorm[in[i].first])
      linNorm[in[i].first] = cnt++;
  }
  for (int i = 1; i <= n; i++) {
    if (!colNorm[in[i].second])
      colNorm[in[i].second] = cnt++;
    int x = linNorm[in[i].first];
    int y = colNorm[in[i].second];
    G[x].push_back({y, i});
    G[y].push_back({x, i});
  }
  nodeCnt = cnt - 1;
  for (int i = 1; i <= nodeCnt; i++) {
    if (!lev[i]) {
      lev[i] = 1;
      DFS(i);
    }
  }
  int ansCnt = 0;
  for (int i = 1; i <= nodeCnt; i++)
    for (int j = 0; j < (int)ans[i].size() - 1; j += 2)
      ansCnt++;
  cout << ansCnt << "\n";
  for (int i = 1; i <= nodeCnt; i++)
    for (int j = 0; j < (int)ans[i].size() - 1; j += 2) {
      cout << ans[i][j] << " " << ans[i][j + 1] << "\n";
      vf[ans[i][j]] = vf[ans[i][j + 1]] = true;
    }
  int ind = 0;
  for (int i = 1; i <= n; i++) {
    if (!vf[i]) {
      ind++;
      cout << i << (ind % 2 == 1 ? " " : "\n");
    }
  }
}

int main(){
  for (cin >> t; t; t--)
    solve();
  return 0;
}