#include<bits/stdc++.h>
using namespace std;

using pii = pair<int, int>;
const int N = 2e5+5;
int t, n;
struct Pt{int x, y;}pt[N];
vector<int> hshx, hshy;
vector<pii> G[N*2];
int dfn[N*2], idx;
vector<pii> ans;
int anscnt;

int dfs(int x, int fid){
    // printf("dfs(x=%d)\n", x);
    dfn[x]=++idx;
    vector<int> unpair;
    for (auto [v, id] : G[x]){
        if (id==fid) continue;
        if (dfn[v]>dfn[x]) continue;
        if (dfn[v]!=-1 && dfn[v]<dfn[x]){
            unpair.push_back(id);
            continue;
        }
        int lft=dfs(v, id);
        if (lft>-1) ans.emplace_back(lft, id), ++anscnt;
        else unpair.push_back(id);
    }
    for (int i=0; i+1<(int)unpair.size(); i+=2){
        ans.emplace_back(unpair[i], unpair[i+1]), ++anscnt;
    }
    return (unpair.size()%2==1 ? unpair.back() : -1);
}

void solve(){
    cin >> n;
    hshx.clear(), hshy.clear(); ans.clear(); anscnt=0, idx=0;
    for (int i=1; i<=2*n; ++i){
        cin >> pt[i].x >> pt[i].y;
        hshx.push_back(pt[i].x); hshy.push_back(pt[i].y);
    }
    sort(hshx.begin(), hshx.end());
    hshx.erase(unique(hshx.begin(), hshx.end()), hshx.end());
    sort(hshy.begin(), hshy.end());
    hshy.erase(unique(hshy.begin(), hshy.end()), hshy.end());
    for (int i=1; i<=(int)hshx.size(); ++i) G[i].clear(), dfn[i]=-1;
    for (int i=1; i<=(int)hshy.size(); ++i) G[i+N].clear(), dfn[i+N]=-1;
    for (int i=1; i<=2*n; ++i){
        pt[i].x = lower_bound(hshx.begin(), hshx.end(), pt[i].x)-hshx.begin()+1;
        pt[i].y = lower_bound(hshy.begin(), hshy.end(), pt[i].y)-hshy.begin()+1;
        G[pt[i].x].emplace_back(N+pt[i].y, i);
        G[N+pt[i].y].emplace_back(pt[i].x, i);
    }
    // for (int i=1; i<=(int)hshx.size(); ++i){
    //     printf("i=%d:", i); for (auto [v, id] : G[i]) printf("(v=%d id=%d)", v, id); puts("");
    // }
    vector<int> unpair;
    for (int i=1; i<=hshx.size(); ++i){
        if (-1==dfn[i]){
            int lft = dfs(i, -1);
            if (lft>-1) unpair.push_back(lft);
        }
    }
    // assert(unpair.size()%2==0);
    for (int i=0; i+1<unpair.size(); i+=2){
        ans.emplace_back(unpair[i], unpair[i+1]);
    }
    cout << anscnt << '\n';
    for (auto [a, b] : ans) cout << a << ' ' << b << '\n';
    // puts("11111");
}
signed main(){
    ios::sync_with_stdio(0); cin.tie(0);
    cin >> t; while (t--) solve();
    return 0;
}