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



int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int T;
    cin >> T;
    while (T--) {
        int n;
        cin >> n;
        map<int, int> mpx, mpy;
        int tot = 0;

        auto get = [&](int x, map<int, int> &mp) {
            if (mp.find(x) == mp.end()) {
                mp[x] = tot++;
            }
            return mp[x];
        };

        vector<vector<pair<int, int>>> e(4 * n);
        for (int i = 0; i < 2 * n; ++i) {
            int x, y;
            cin >> x >> y;
            x = get(x, mpx);
            y = get(y, mpy);
            e[x].push_back({y, i});
            e[y].push_back({x, i});
        }

        vector<bool> vis(tot);
        vector<bool> in(tot);

        vector<int> p(2 * n);
        vector<int> bin;

        int ans = 0;
        auto dfs = [&](auto &&self, int u, int fa) -> int {
            in[u] = true;
            vis[u] = true;
            int pre = -1;
            for (auto [v, id] : e[u]) {
                if (v == fa) {
                    continue;
                }
                if (vis[v]) {
                    if (!in[v]) {
                        if (pre != -1) {
                            p[pre] = id;
                            p[id] = pre;
                            pre = -1;
                            ++ans;
                        } else {
                            pre = v;
                        }
                    }
                } else {
                    int t = self(self, v, u);
                    if (t != -1) {
                        p[t] = id;
                        p[id] = t;
                        ++ans;
                    } else {
                        if (pre != -1) {
                            p[pre] = id;
                            p[id] = pre;
                            pre = -1;
                            ++ans;
                        } else {
                            pre = id;
                        }
                    }
                }
            }
            in[u] = false;
            return pre;
        };

        for (int i = 0; i < tot; ++i) {
            if (!vis[i]) {
                int t = dfs(dfs, i, -1);
                if (t != -1) {
                    bin.push_back(t);
                }
            }
        }

        for (int i = 0; i < (int)bin.size(); i += 2) {
            p[bin[i]] = bin[i + 1];
            p[bin[i + 1]] = bin[i];
        }

        cout << ans << '\n';
        for (int i = 0; i < 2 * n; ++i) {
            if (i < p[i]) {
                cout << i + 1 << ' ' << p[i] + 1 << '\n';
            }
        }
    }
    return 0;
}