#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <functional>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using Int = long long;

template <class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &a) { return os << "(" << a.first << ", " << a.second << ")"; };
template <class T> ostream &operator<<(ostream &os, const vector<T> &as) { const int sz = as.size(); os << "["; for (int i = 0; i < sz; ++i) { if (i >= 256) { os << ", ..."; break; } if (i > 0) { os << ", "; } os << as[i]; } return os << "]"; }
template <class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cerr << *i << " "; cerr << endl; }
template <class T> bool chmin(T &t, const T &f) { if (t > f) { t = f; return true; } return false; }
template <class T> bool chmax(T &t, const T &f) { if (t < f) { t = f; return true; } return false; }


struct LineGraphMaxMatch {
  int n, m;
  vector<int> as, bs;
  vector<vector<int>> g;

  vector<int> to;

  LineGraphMaxMatch(int n_ = 0) : n(n_), m(0), g(n) {}
  void ae(int u, int v) {
    assert(0 <= u); assert(u < n);
    assert(0 <= v); assert(v < n);
    g[u].push_back(m);
    g[v].push_back(m);
    ++m;
    as.push_back(u);
    bs.push_back(v);
  }

  vector<int> vis;
  vector<int> used;
  int dfs(int u) {
    vis[u] = 1;
    int ret = -1;
    for (const int i : g[u]) if (!used[i]) {
      const int v = as[i] ^ bs[i] ^ u;
      used[i] = 1;
      int tmp = i;
      if (!vis[v]) {
        const int res = dfs(v);
        if (~res) {
          to[tmp] = res; to[res] = tmp; tmp = -1;
        }
      }
      if (~tmp) {
        if (~ret) {
          to[ret] = tmp; to[tmp] = ret; ret = -1;
        } else {
          ret = tmp;
        }
      }
    }
    return ret;
  }
  int run() {
    to.assign(m, -1);
    vis.assign(n, 0);
    used.assign(m, 0);
    for (int u = 0; u < n; ++u) if (!vis[u]) dfs(u);
    int ma = 0;
    for (int i = 0; i < m; ++i) if (i < to[i]) ++ma;
    return ma;
  }
};


int N;
vector<int> X, Y;

int main() {
  for (int numCases; ~scanf("%d", &numCases); ) { for (int caseId = 1; caseId <= numCases; ++caseId) {
    scanf("%d", &N);
    X.resize(2*N);
    Y.resize(2*N);
    for (int i = 0; i < 2*N; ++i) {
      scanf("%d%d", &X[i], &Y[i]);
    }
    
    auto xs = X;
    auto ys = Y;
    sort(xs.begin(), xs.end());
    sort(ys.begin(), ys.end());
    for (int i = 0; i < 2*N; ++i) {
      X[i] = lower_bound(xs.begin(), xs.end(), X[i]) - xs.begin();
      Y[i] = lower_bound(ys.begin(), ys.end(), Y[i]) - ys.begin();
    }
    
    LineGraphMaxMatch lg(2*N + 2*N);
    for (int i = 0; i < 2*N; ++i) {
      lg.ae(X[i], 2*N + Y[i]);
    }
    const int ans = lg.run();
    vector<int> remain;
    for (int i = 0; i < 2*N; ++i) if (!~lg.to[i]) {
      remain.push_back(i);
    }
    
    printf("%d\n", ans);
    for (int i = 0; i < 2*N; ++i) if (i < lg.to[i]) {
      printf("%d %d\n", i + 1, lg.to[i] + 1);
    }
    for (int h = 0; h < (int)remain.size(); h += 2) {
      printf("%d %d\n", remain[h] + 1, remain[h + 1] + 1);
    }
  }
#ifndef LOCAL
  break;
#endif
  }
  return 0;
}