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

#define int long long
#define pii pair<int, int>
#define ff first
#define ss second
const int MAXN = 2e5 + 5;

int n, cnt, tot;
map<pii, int> mp;
vector<pii> adj[2 * MAXN];
pii ans[2 * MAXN];
bool vis[2 * MAXN], matched[2 * MAXN];

int getid(pii pr) {
    if(mp.find(pr) == mp.end()) {
        mp[pr] = ++cnt;
    }
    return mp[pr];
}

bool magic(int u, int p, int edge) {    // true if par edge wasn't used
    vis[u] = true;
    vector<int> children;

    for(auto pr : adj[u]) {
        int v = pr.ff, i = pr.ss;

        if(v == p) continue;
        if(!vis[v]) {
            if(magic(v, u, i)) children.push_back(i);
        }
        else if(!matched[i]) children.push_back(i);
    }

    // cout << "u: ";
    // for(auto child : children) cout << child << " ";
    // cout << endl;

    for(int i = 1; i < children.size(); i += 2) {
        ans[++tot] = {children[i - 1], children[i]};
        matched[children[i - 1]] = matched[children[i]] = true;
    }

    if(children.size() % 2 && p != -1) {
        ans[++tot] = {children.back(), edge};
        matched[children.back()] = matched[edge] = true;
        return false;
    }
    return true;
}

void solve() {
    cin >> n;

    mp.clear();
    for(int i = 0; i <= max(2 * cnt, 2 * n); ++i) {
        adj[i].clear(); vis[i] = matched[i] = false;
    }
    cnt = tot = 0;

    for(int i = 1; i <= 2 * n; ++i) {
        int a, b; cin >> a >> b;

        a = getid({a, 0}); b = getid({b, 1});
        // cout << a << " " << b << endl;
        adj[a].push_back({b, i});
        adj[b].push_back({a, i});
    }

    // for(int i = 1; i <= cnt; ++i) {
    //     cout << i << ": ";
    //     for(auto pr : adj[i]) cout << pr.ss << " ";
    //     cout << endl;
    // }

    for(int i = 1; i <= cnt; ++i) {
        if(vis[i]) continue;
        magic(i, -1, 0);
    }

    cout << tot << endl;
    for(int i = 1; i <= tot; ++i) cout << ans[i].ff << " " << ans[i].ss << endl;
    
    int num = 0;
    for(int i = 1; i <= 2 * n; ++i) {
        if(!matched[i]) {
            cout << i << " ";
            if(num++ % 2) cout << endl;
        }
    } 
}

signed main() {
    cin.tie(0)->sync_with_stdio(false);
    int t; cin >> t;
    while(t--) solve();
}