#include<bits/stdc++.h>
#define ll long long
#define ull unsigned long long
#define lowbit(i) (i&-i)
#define rand() (myrand())
using namespace std;
mt19937 myrand(time(0));
const int maxn = 5e5 + 5;
int n, _, cx[maxn], cy[maxn], vis[maxn];
struct node{
	int x, y;
}a[maxn];
void solve() {
	cin >> n;
	n <<= 1;
	vector<int>px, py;
	vector<pair<int, int> >ans;
	priority_queue<pair<int, int> >qx, qy;
	map<pair<int, int>, int>id;	
	int num = 0;
	for (int i = 1 ; i <= n ; ++i) {
		int x, y;
		cin >> x >> y;
		a[i].x = x, a[i].y = y;
		id[{x, y}] = i;
		vis[i] = 0;
		if (!cx[x]) px.push_back(x);
		if (!cy[y]) py.push_back(y);
		cx[x]++, cy[y]++;
	}
	for (auto x : px) qx.push({cx[x], x});
	for (auto y : py) qy.push({cy[y], y});
	while(1) {
		pair<int, int>t1, t2, t, p1, p2;
		int maxh = -1, f = 0;
		if (qx.size() >= 2 && qy.size() >= 1) {
			t1 = qx.top();qx.pop();
			t2 = qx.top();qx.pop();
			t = qy.top();qy.pop();
			if (t.first - 2 >= 0) {
				int last = maxh;
				maxh = max(maxh, min({t1.first - 1, t2.first - 1, t.first - 2}));
				if (maxh > last) {
					p1.second = p2.second = t.second;
					p1.first = t1.second;
					p2.first = t2.second;
				}
				f = 1;
			}
			qx.push(t1);	
			qx.push(t2);
			qy.push(t);
		}
		if (qx.size() >= 1 && qy.size() >= 2) {
			t1 = qy.top();qy.pop();
			t2 = qy.top();qy.pop();
			t = qx.top();qx.pop();
			if (t.first - 2 >= 0) {
				int last = maxh;
				maxh = max(maxh, min({t1.first - 1, t2.first - 1, t.first - 2}));
				if (maxh > last) {
					p1.first = p2.first = t.second;
					p1.second = t1.second;
					p2.second = t2.second;
				}	
				f = 2;
			}	
			qy.push(t1);	
			qy.push(t2);
			qx.push(t);	
		}
		if (!f) break;
		num++;
		if (f == 1) {
			t1 = qx.top();qx.pop();
			t2 = qx.top();qx.pop();
			t = qy.top();qy.pop();
			t1.first--;
			t2.first--;
			t.first -= 2;
			if (t1.first > 0) qx.push(t1);	
			if (t2.first > 0) qx.push(t2);
			if (t.first > 0) qy.push(t);		
		}
		else {
			t1 = qy.top();qy.pop();
			t2 = qy.top();qy.pop();
			t = qx.top();qx.pop();		
			t1.first--;
			t2.first--;
			t.first -= 2;
			if (t1.first > 0) qy.push(t1);	
			if (t2.first > 0) qy.push(t2);
			if (t.first > 0) qx.push(t);	
		}
		ans.push_back({id[p1], id[p2]});
		vis[id[p1]] = vis[id[p2]] = 1;
	}
	vector<int>res;
	for (int i = 1 ; i <= n ; ++i) {
		if (!vis[i]) res.push_back(i);
	}
	for (int i = 0 ; i < res.size() ; i += 2) ans.push_back({res[i], res[i + 1]});
	cout << num << '\n';
	for (auto [x, y] : ans) cout << x << ' ' << y << '\n';
	for (int i = 1 ; i <= n ; ++i) cx[a[i].x] = cy[a[i].y] = 0;
}
signed main(void) {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cin >> _;
	while(_--) solve();
	return 0;
}