// Source: https://usaco.guide/general/io

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

int X, Y;

vector<pair<int, int>> ans;
vector<int> leftover;

void dfs(int curr, int prev_edge, vector<vector<pair<int, int>>> &adj, vector<bool> &edges, vector<bool> &visited) {
	visited[curr] = true;

	vector<int> edges2;

	for (auto next : adj[curr]) {
		if (!visited[next.first] && (next.second != prev_edge)) {
			dfs(next.first, next.second, adj, edges, visited);
		}
	}

	for (auto next : adj[curr]) {
		if (!edges[next.second] && (next.second != prev_edge)) {
			edges2.push_back(next.second);
		}
	}

	if (edges2.size() % 2) { // needed for optimal solution
    	edges2.push_back(prev_edge);
  	}

  	for (int i = 0; i < (int) edges2.size(); i += 2) {
    	if (edges2[i] != -1 && edges2[i + 1] != -1){
			edges[edges2[i]] = true; edges[edges2[i + 1]] = true;

      		ans.push_back({edges2[i], edges2[i + 1]});
    	} else {
			// these are the ones you can't do
			if (edges2[i] == -1) {
				leftover.push_back(edges2[i + 1]);
			} else {
				leftover.push_back(edges2[i]);
			}
		}
  	}
}

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

	vector<pair<pair<int, int>, int>> arr(2 * N); ans.clear(); leftover.clear();

	map<int, int> xs, ys;

	for (int i = 0; i < 2 * N; i++) {
		cin >> arr[i].first.first >> arr[i].first.second; arr[i].second = i;
		xs[arr[i].first.first] = 0;
		ys[arr[i].first.second] = 0;
	}

	X = 0, Y = 0;

	for (auto it = xs.begin(); it != xs.end(); it = next(it)) {
		it->second = X++;
	}

	for (auto it = ys.begin(); it != ys.end(); it = next(it)) {
		it->second = Y++;
	}

	vector<vector<pair<int, int>>> adj(X + Y);

	// edges in tree is stored in the edges array
	vector<bool> edges(2 * N, false);
	vector<bool> visited(X + Y, false);

	for (int i = 0; i < 2 * N; i++) {
		int a = xs[arr[i].first.first], b = ys[arr[i].first.second]; b += X;

		adj[a].push_back({b, i});
		adj[b].push_back({a, i});
	}

	// dfs tree
	for (int i = 0; i < X + Y; i++) {
		if (!visited[i]) {
			dfs(i, -1, adj, edges, visited);
		}
	}

	cout << (int) ans.size() << "\n";
	for (auto p : ans) {
		cout << p.first + 1 << " " << p.second + 1 << "\n";
	}

	// L
	for (int i = 0; i < (int) leftover.size(); i += 2) {
		cout << leftover[i] + 1 << " " << leftover[i + 1] + 1 << "\n";
	}
}

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

	int T; cin >> T;

	while (T--) {
		solve();
	}
}