#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef tuple<int, int, int> tii;
typedef tuple<ll, ll, ll> tll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<ll> vl;
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define nl "\n"
#define all(v) v.begin(),v.end()
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define sz(x) (int)(x).size()

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count()); 
const int maxn = 2e5 + 5;

int n, m, q, k, t, a, b, c;
int x[maxn], y[maxn];
int p[2 * maxn];
int p1[2 * maxn];

vector<pii> v[2 * maxn];
bool vis[2 * maxn];
bool used[maxn];
int dp[2 * maxn];
bool odd[maxn];
vector<pii> ans;
int cnt = 0;

void pairup(int x, int y, int par) {
	vector<int> nodes;
	while (x != par) {
		nodes.pb(p1[x]);
		x = p[x];
	}
	vector<int> nodes1;
	while (y != par) {
		nodes1.pb(p1[y]);
		y = p[y];
	}
	reverse(all(nodes1));
	for (auto it : nodes1) nodes.pb(it);
	for (int i = 1; i < nodes.size(); i += 2) {
		cnt++;
		int a = nodes[i - 1], b = nodes[i];
		ans.eb(a, b);
		used[a] = used[b] = true;
	}
}

void dfs(int x, int par, int ed) {
//	cout << x << nl;
	p[x] = par;
	p1[x] = ed;
	vis[x] = true;
	vector<int> st;
	for (auto [a, b] : v[x]) {
		if (vis[a]) continue;
		dfs(a, x, b);
		if (dp[a] != 0) st.pb(dp[a]);
	}
	if (x <= n && odd[x]) {
		st.pb(x);
	}
	for (int i = 1; i < st.size(); i += 2) {
		pairup(st[i - 1], st[i], x);
	}
	dp[x] = 0;
	if (st.size() % 2) {
		dp[x] = st.back();
	}
}

int main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	cin >> t;
	while (t--) {
		cin >> n;
		n *= 2;
		cnt = 0;
		for (int i = 1; i <= n; i++) {
			used[i] = false;
			vis[i] = vis[i + n] = false;
			v[i].clear();
			v[i + n].clear();
			dp[i] = dp[i + n] = 0;
		}
		ans.clear();
		vector<int> vx, vy;
		for (int i = 1; i <= n; i++) {
			cin >> x[i] >> y[i];
			vx.pb(x[i]), vy.pb(y[i]);
		}
		sort(all(vx)), sort(all(vy));
		vector xs(n + 1, vector<int>());
		vector ys(n + 1, vector<int>());
		for (int i = 1; i <= n; i++) {
			x[i] = lower_bound(all(vx), x[i]) - vx.begin() + 1;
			xs[x[i]].pb(i);
			y[i] = lower_bound(all(vy), y[i]) - vy.begin() + 1;
			ys[y[i]].pb(i);
			v[x[i]].eb(y[i] + n, i);
			v[y[i] + n].eb(x[i], i);
		}
		
		for (int i = 1; i <= n; i++) {
			if (xs[i].size() % 2) odd[i] = true;
			else odd[i] = false;
		}
		
		for (int i = 1; i <= 2 * n; i++) {
			if (!vis[i]) {
		//		cout << "start\n";
				dfs(i, 0, 0);
			}
		}
		
		for (int i = 1; i <= n; i++) {
			vector<int> tmp;
			for (auto it : xs[i]) {
				if (!used[it]) tmp.pb(it);
			}
			for (int j = 1; j < tmp.size(); j += 2) {
				cnt++;
				ans.eb(tmp[j - 1], tmp[j]);
				used[tmp[j - 1]] = used[tmp[j]] = true;
			}
		}
		
		vector<int> tmp;
		for (int i = 1; i <= n; i++) {
			if (!used[i]) tmp.pb(i);
		}
		for (int j = 1; j < tmp.size(); j += 2) {
			ans.eb(tmp[j - 1], tmp[j]);
			used[tmp[j - 1]] = used[tmp[j]] = true;
		}
		
		cout << cnt << nl;
		for (auto [a, b] : ans) {
			cout << a << " " << b << nl;
		}
	}
}