#include <bits/stdc++.h>
using namespace std;
#define sz(x) (int)(x).size()
#define rep(i, l, r) for (int i = l; i < (r); i++)
#define all(x) begin(x), end(x)


using ll = long long;
using vi = vector<int>;

using pii = pair<int, int>;



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

    vi x(n), y(n);rep(i, 0, n)cin >> x[i] >> y[i];
    vi xc, yc;
    rep(i, 0, n)xc.emplace_back(x[i]), yc.emplace_back(y[i]);
    sort(all(xc));sort(all(yc));
    xc.erase(unique(all(xc)), xc.end());
    yc.erase(unique(all(yc)), yc.end());

    rep(i, 0, n)x[i] = lower_bound(all(xc), x[i]) - xc.begin();
    rep(i, 0, n)y[i] = lower_bound(all(yc), y[i]) - yc.begin();
    // rep(i, 0, n)cout << x[i] << " ";cout << endl;
    
    vector<vector<pii>> g(n*2);
    vi deg(n*2, 0);rep(i, 0, n)deg[x[i]]++, deg[y[i]+n]++, g[x[i]].emplace_back(y[i]+n, i), g[y[i]+n].emplace_back(x[i], i);
    vi used(n, 0);
    queue<int> a;
    rep(i, 0, n*2)if(deg[i] == 1)a.push(i);
    vector<pii> ans;
    rep(i, 0, n * 2) {
        while(sz(a)) {
            int v = a.front();a.pop();
            if (deg[v] == 0)continue;
            // cout << v << endl;
            auto [u, j] = [&]->pii{
                for(auto [u, j] : g[v])if(used[j]==0)return {u, j};}();
            // cout << v << " " << u << " " << j << endl;
            deg[u] -= 1;
            deg[v] -= 1;
            used[j] = 1;
            if (deg[u]) {
                auto [w, k] = [&]->pii{
                for(auto [w, k] : g[u])if(used[k]==0)return {w, k};}();
                deg[w] -= 1;
                deg[u] -= 1;
                used[k] = 2;
                used[j] = 2;
                ans.emplace_back(j, k);
                if (deg[w] == 1)a.push(w);
            }
            if (deg[u] == 1)a.push(u);
            
        }
        if (deg[i] > 0)a.push(i);
    }

    int last = -1;
    cout << sz(ans) << endl;
    rep(i, 0, n) {
        if (used[i] == 2)continue;
        if (last == -1) last = i;
        else {
            ans.emplace_back(last, i);
            last = -1;
        }
    }
    for (auto [x, y]:ans)cout << x+1 << " " << y+1 << endl;
}

int main() {
    int T;cin >> T;
    while (T--)solve();
}