/**
 * @file qoj6414_1.cpp
 * @author n685
 * @brief
 * @date 2023-08-12
 *
 *
 */
#include <bits/stdc++.h>

#ifdef LOCAL
std::ifstream cin;
std::ofstream cout;
using std::cerr;
#else
using std::cin;
using std::cout;
#define cerr                                                                   \
  if (false)                                                                   \
  std::cerr
#endif

template <class T> struct CC {
  std::vector<T> val;
  void push(T a) { val.push_back(a); }
  void init() {
    std::sort(val.begin(), val.end());
    val.erase(std::unique(val.begin(), val.end()), val.end());
  }
  int operator[](T a) const {
    return static_cast<int>(std::distance(
        val.begin(), std::lower_bound(val.begin(), val.end(), a)));
  }
  int size() const { return static_cast<int>(val.size()); }
};

void solve() {
  int n;
  cin >> n;

  CC<int> cx, cy;
  std::vector<int> x(2 * n), y(2 * n);
  for (int i = 0; i < 2 * n; ++i) {
    cin >> x[i] >> y[i];
    cx.push(x[i]);
    cy.push(y[i]);
  }
  cx.init(), cy.init();
  int sz = cx.size() + cy.size();
  std::vector<std::vector<std::pair<int, int>>> adj(sz);
  for (int i = 0; i < 2 * n; ++i) {
    adj[cx[x[i]]].emplace_back(i, cx.size() + cy[y[i]]);
    adj[cx.size() + cy[y[i]]].emplace_back(i, cx[x[i]]);
  }

  std::vector<bool> vis(sz);
  std::vector<std::pair<int, int>> ans;
  std::vector<int> np;

  std::vector<int> d(sz);
  auto dfs = [&](auto &self, int node, int par) -> bool {
    vis[node] = true;
    std::vector<int> e;
    int p = -1;
    for (auto [ind, i] : adj[node]) {
      if (i == par) {
        p = ind;
      } else if (vis[i]) {
        if (d[node] < d[i])
          e.push_back(ind);
      } else {
        d[i] = d[node] + 1;
        if (!self(self, i, node))
          e.push_back(ind);
      }
    }
    for (int i = 0; i + 1 < (int)e.size(); i += 2)
      ans.emplace_back(e[i], e[i + 1]);
    if ((int)e.size() % 2 == 1) {
      if (p == -1)
        np.push_back(e.back());
      else
        ans.emplace_back(e.back(), p);
      return true;
    }
    return false;
  };

  for (int i = 0; i < sz; ++i) {
    if (!vis[i]) {
      dfs(dfs, i, -1);
    }
  }
  cout << (int)ans.size() << '\n';
  for (int i = 0; i + 1 < (int)np.size(); i += 2)
    ans.emplace_back(np[i], np[i + 1]);
  for (auto [a, b] : ans)
    cout << a + 1 << ' ' << b + 1 << '\n';
}

int main() {
#ifdef LOCAL
  cin.open("input.txt");
  cout.rdbuf()->pubsetbuf(0, 0);
  cout.open("output.txt");
#else
  cin.tie(nullptr)->sync_with_stdio(false);
#endif

  int t;
  cin >> t;
  while (t--)
    solve();
}