#include <bits/stdc++.h>
using namespace std;
using namespace chrono;

using ll = long long;
using ull = unsigned long long;

string to_string(const string &s) {
  return '"' + s + '"';
}

string to_string(bool b) {
  return b ? "true" : "false";
}

template <typename A, typename B>
string to_string(const pair<A, B> &p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename T>
string to_string(const T &v) {
  string s = "{";
  bool first = true;
  for (const auto &it : v) {
    if (!first)
      s += ", ";
    else
      first = false;
    s += to_string(it);
  }
  return s += "}";
}

void debug_out() {
  cerr << endl;
}

template <typename T, typename... Args>
void debug_out(const T &first, const Args&... rest) {
  cerr << to_string(first) << " ";
  debug_out(rest...);
}

#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

auto startTime = high_resolution_clock::now();
int get_time() {
  auto stopTime = chrono::high_resolution_clock::now();
  auto duration = duration_cast<milliseconds>(stopTime - startTime);
  return duration.count(); // in ms
}

const int DIM = 2e5 + 5;

vector<pair<int, int>> v[8 * DIM];
int wh[2 * DIM];
bool viz[8 * DIM];
int nr = 0;

void dfs(int nod, int papa = 0) {
  nr += v[nod].size();
  viz[nod] = 1;

  int last = 0, wp = 0;
  for (auto it : v[nod]) {
    if (it.first == nod) {
      wp = it.second;
      continue;
    }

    if (!viz[it.first])
      dfs(it.first, nod);

    if (!wh[it.second]) {
      if (!last)
        last = it.second;
      else {
        wh[it.second] = last;
        wh[last] = it.second;
        last = 0;
      }
    }
  }

  if (last && wp) {
    wh[last] = wp;
    wh[wp] = last;
  }
}

void solve() {
  int n;
  cin >> n;
  vector<int> x(2 * n + 1, 0);
  vector<int> y(2 * n + 1, 0);

  int n_x = 0, n_y = 0;
  map<int, int> fx, fy;
  for (int i = 1; i <= 2 * n; ++i) {
    cin >> x[i] >> y[i];
    if (!fx.count(x[i]))
      fx[x[i]] = ++n_x;
    if (!fy.count(y[i]))
      fy[y[i]] = ++n_y;
  }

  for (int i = 1; i <= 2 * n; ++i) {
    wh[i] = 0;
    v[fx[x[i]]].push_back({fy[y[i]] + n_x, i});
    v[fy[y[i]] + n_x].push_back({fx[x[i]], i});
  }

  int ans = 0;
  for (int i = 1; i <= n_x; ++i) {
    if (!viz[i]) {
      nr = 0;
      dfs(i);
      ans += nr / 4;
    }
  }

  cout << ans << '\n';

  vector<int> bads;
  for (int i = 1; i <= 2 * n; ++i) {
    if (!wh[i])
      bads.push_back(i);
    else if (i < wh[i]) {
      cout << i << " " << wh[i] << '\n';
    }
  }

  while (!bads.empty()) {
    int x = bads.back();
    bads.pop_back();
    cout << x << " ";
    x = bads.back();
    bads.pop_back();
    cout << x << "\n";
  }

  for (int i = 1; i <= 8 * n; ++i) {
    viz[i] = 0;
    v[i].clear();
  }
}

int main() {
  cin.tie(NULL);
  ios_base::sync_with_stdio(false);

  int t = 1;
  cin >> t;
  while (t--)
    solve();

  return 0;
}