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

using ll = long long;
using pii = pair<int, int>;

void solve() {
	int n;
	cin >> n;
	n = n * 2;

	vector<array<int, 2>> a(n);
	vector<int> x, y;
	for (int i = 0; i < n; i++) {
		cin >> a[i][0] >> a[i][1];
		x.push_back(a[i][0]);
		y.push_back(a[i][1]);
	}
	sort(begin(x), end(x));
	sort(begin(y), end(y));
	x.resize(unique(begin(x), end(x)) - begin(x));
	y.resize(unique(begin(y), end(y)) - begin(y));
	
	int nx = (int)x.size();
	int ny = (int)y.size();
	vector<vector<pii>> g(nx + ny);
	for (int i = 0; i < n; i++) {
		a[i][0] = lower_bound(begin(x), end(x), a[i][0]) - begin(x);
		a[i][1] = lower_bound(begin(y), end(y), a[i][1]) - begin(y);
		g[a[i][0]].push_back({nx + a[i][1], i + 1});
		g[nx + a[i][1]].push_back({a[i][0], i + 1});
	}

	vector<bool> used(n + 1);
	vector<pii> ans;

	auto add = [&](int x, int y) {
		used[x] = true;
		used[y] = true;
		ans.push_back({x, y});
	};

	int cnt = 0;

	vector<bool> vis(nx + ny);
	vector<int> dep(nx + ny);
	auto dfs = [&](auto &&dfs, int cur, int prev, int preveg, int d) -> bool {
		dep[cur] = d;
		vis[cur] = true;
		vector<int> ueg;
		for (auto [nxt, eg]: g[cur]) {
			if (nxt == prev) continue;
			if (!vis[nxt]) {
				bool fl = dfs(dfs, nxt, cur, eg, d + 1);
				if (fl) { // not used
					ueg.push_back(eg);
				}
			}
			else {
				if (dep[nxt] > dep[cur]) {
					ueg.push_back(eg);
				}
			}
		}
		int sz = (int)ueg.size();
		if (sz % 2 == 0) {
			for (int i = 0; i < sz; i += 2) {
				++cnt;
				add(ueg[i], ueg[i + 1]);
			}
			return true;
		}
		else {
			for (int i = 1; i < sz; i += 2) {
				++cnt;
				add(ueg[i], ueg[i + 1]);
			}
			if (prev != -1) {
				++cnt;
				add(ueg[0], preveg);
				return false;
			}
			return true;
		}
	};
	for (int i = 0; i < nx + ny; i++) {
		if (!vis[i]) dfs(dfs, i, -1, -1, 1);
	}

	int prev = -1;
	for (int i = 1; i <= n; i++) {
		if (!used[i]) {
			if (prev == -1) prev = i;
			else {
				add(prev, i);
				prev = -1;
			}
		}
	}

	cout << cnt << "\n";
	for (auto [x, y]: ans) {
		cout << x << " " << y << "\n";
	}

}

int main() {
	cin.tie(0)->sync_with_stdio(0);
	int t;
	cin >> t;
	while (t--) {
		solve();
	}	
}