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

#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

int main() {
	cin.tie(0)->sync_with_stdio(0);
	cin.exceptions(cin.failbit);

  int tt;
  cin >> tt;
  while(tt--) {
    int n;
    cin >> n;
    n *= 2;
    vector<int> xs, ys;
    vector<pii> pts;
    rep(i, 0, n) {
      int x, y;
      cin >> x >> y;
      pts.push_back({x, y});
      xs.push_back(x), ys.push_back(y);
    }
    sort(all(xs));
    sort(all(ys));
    xs.erase(unique(all(xs)), end(xs));
    ys.erase(unique(all(ys)), end(ys));

    auto xget = [&](int x)->int {
      return lower_bound(xs.begin(), xs.end(), x) - xs.begin();
    };
    auto yget = [&](int y)->int {
      return sz(xs) + (lower_bound(ys.begin(), ys.end(), y) - ys.begin());
    };

    int c = sz(xs) + sz(ys);
    vector<vector<pair<int,int>>> g(c);
    for (int i =0; i < sz(pts); i++) {
      auto& [x, y] = pts[i];
      int X = xget(x), Y = yget(y);
      g[X].push_back({Y, i});
      g[Y].push_back({X, i});
    }


    vi vis(c), taken(n);
    vector<pii> ans;
    for (int i =0; i < c; i++) {
      if (vis[i]) continue;
      auto dfs = [&](auto& self, int s)->int {
        vis[s] = 1;
        /* if (s < xs.size()) cout << xs[s] << " "; */
        /* else cout << ys[s - sz(xs)] << " "; */
        /* cout << endl; */
        int ret = -1;
        for (auto& [u, idx] : g[s]) {
          if (vis[u] == 1) continue; // back edge
          int nw = idx;
          if (!vis[u]) {
            int tmp = self(self, u);
            if (tmp != -1) {
              taken[nw] = taken[tmp] = 1;
              ans.push_back({nw, tmp});
              nw = -1;
            }
          }
          if (nw != -1) {
            if (ret != -1) {
              taken[ret] = taken[nw] = 1;
              ans.push_back({ret, nw});
              ret = -1;
            } else ret = nw;
          }
        }
        vis[s] = 2;
        return ret;
      };
      dfs(dfs, i);
    }
    int friends = ans.size();
    vi not_taken;
    int last = -1;
    for (int i =0; i < n; i++) {
      if (!taken[i]) {
        if (last == -1) last=  i;
        else {
          ans.push_back({last, i});
          last = -1;
        }
      }
    }
    assert(last == -1);
    cout << friends << '\n';
    for (auto& [x, y] : ans) {
      cout << x+1 << " " << y+1 << '\n';
    }
  }
}