/*
3
2
10 10
10 1
1 10
1 1
2
0 0
0 1
0 2
0 3
2
0 0
1 1
2 2
3 3
*/
#include <iostream>
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define vi vector<int>
#define vl vector<ll>
#define vb vector<bool>
#define vii vector<vector<int>>
#define vll vector<vector<ll>>
#define pii pair<int, int>
#define pil pair<int, ll>

const int maxn = 200005;
int inp[maxn][2];

int n; int prs; 
vector<vb> seen; vb used; vi p; //pairing for certain edge
vector<vector<vector<pii>>> x2y; //stores connection, index of edge
void dfs(int c, int x, int prt){ //whether you need to make a pair with your parent or not
    //make sure you dont accidentally pair an edge with itself --> only visit an edge if the depths are correct (depth of first value is less than depth of second value - that means this edge was taken)
    for(pii edge : x2y[x][c]){ 
        if(!seen[1-x][edge.first]){
            seen[1-x][edge.first] = true; 
            dfs(edge.first, 1-x, edge.second);
        }
    }
    //cout << "at " << c << " coord " << x << endl; 
    int pv = -1; 
    for(pii edge : x2y[x][c]){
        if(!used[edge.second] && edge.second != prt){
            if(pv == -1) pv = edge.second; 
            else{
                prs++; 
                used[edge.second] = used[pv] = true; 
                p[edge.second] = pv; 
                pv = -1; 
            }
        }
    }
    if(pv != -1 && prt != -1 && !used[prt]){
        prs++; 
        used[pv] = used[prt] = true; 
        p[pv] = prt; 
    }
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
	int nc; cin >> nc; 
    for(int cc = 0; cc < nc; cc++){
        cin >> n; n *= 2; 
        prs = 0; 
        seen = vector<vb>(2, vb(n*2+5, false)); used = vb(n, false); p = vi(n, -1);
        x2y = vector<vector<vector<pii>>>(2, vector<vector<pii>>(n*2+5, vector<pii>()));
        map<int, int> comp; 
        for(int a = 0; a < n; a++){
            int f; int s; cin >> f >> s; 
            inp[a][0] = f; inp[a][1] = s; 
            comp[f] = -1; comp[s] = -1; 
        } 
        int ind = 0; 
        for(auto it : comp){
            comp[it.first] = ind++; 
        }
        for(int a = 0; a < n; a++){
            x2y[0][comp[inp[a][0]]].push_back({comp[inp[a][1]], a});
            x2y[1][comp[inp[a][1]]].push_back({comp[inp[a][0]], a});
        }

        for(int a = 0; a < comp.size(); a++){
            for(int b = 0; b < 2; b++){
                if(!seen[b][a]){
                    seen[b][a] = true;
                    dfs(a, b, -1); //if 0 is x
                }
            }
        }
        cout << prs << "\n";
        int pv_u = -1; 
        for(int a = 0; a < n; a++){ //and then pair up unused edges
            if(p[a] != -1) cout << a+1 << " " << p[a]+1 << "\n";
            if(!used[a]){
                if(pv_u != -1){
                    cout << pv_u+1 << " " << a+1 << "\n";
                    pv_u = -1; 
                }
                else pv_u = a; 
            }
        } 
    }
}