#include <iostream>
#include <vector>
#include <map>

using namespace std;

vector <pair <int,int> > graph[300000];
vector <pair <int,int>> pairs;
vector <int> notpaired;

bool hv[300000];
bool hu[300000];

int n;

void makepairs(int p) {
    hv[p] = true;
    for (int i=0; i<graph[p].size(); i++) {
        if (!hv[graph[p][i].first]) {
            makepairs(graph[p][i].first);
        }
    }
    int prev = -1;
    for (int i=0; i<graph[p].size(); i++) {
        //cout << graph[p][i].second << " " << hu[graph[p][i].second] << endl;
        if (!hu[graph[p][i].second]) {
            if (prev==-1) {
                prev = graph[p][i].second;
            }else {
                pairs.push_back({prev,graph[p][i].second});
                //cout << "CREATE PAIR:" << prev << " " << graph[p][i].second << endl;
                hu[prev] = true;
                prev = -1;
                hu[graph[p][i].second] = true;
            }
        }
    }
}

int main() {
    int t;
    cin >> t;
    for (int asdf = 0; asdf<t; asdf++) {
        cin >> n;
        n = n*2;
        for (int i=0; i<n; i++) {
            graph[i].clear();
            hv[i] = false;
            hu[i] = false;
        }
        int x,y;
        pair <int,int> points[n];
        pairs.clear();
        notpaired.clear();
        map <int,int> xp;
        map <int,int> yp;
        for (int i=0; i<n; i++) {
            cin >> points[i].first >> points[i].second;
            xp[points[i].first] = 0;
            yp[points[i].second] = 0;
        }
        int c = 0;
        for (map<int,int>::iterator it=xp.begin(); it!=xp.end(); it++) {
            it->second = c;
            c++;
        }
        c = 0;
        for (map<int,int>::iterator it=yp.begin(); it!=yp.end(); it++) {
            it->second = c;
            c++;
        }
        int u,v;
        for (int i=0; i<n; i++) {
            u = xp[points[i].first];
            v = yp[points[i].second]+n;
            graph[u].push_back({v,i});
            graph[v].push_back({u,i});
        }
        makepairs(0);
        cout << pairs.size() << endl;
        for (int i=0; i<pairs.size(); i++) {
            cout << pairs[i].first+1 << " " << pairs[i].second+1 << endl;
        }
        for (int i=0; i<n; i++) {
            if (!hu[i]) {
                notpaired.push_back(i);
            }
        }
        for (int i=0; i<notpaired.size(); i++) {
            cout << notpaired[i]+1 << " " << notpaired[i+1]+1 << endl;
            i++;
        }
    }
}