#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define pb push_back
#define el << '\n'
#define spe << ' '
#define sp spe <<
#define umap unordered_map
#define uset unordered_set
#define prqu priority_queue
#define fornr(i, j, n) for (int i = j; i < n; i++)
#define forn(i, n) for (int i = 0; i < n; i++)
#define forn1(i, n) for (int i = 1; i <= n; i++)
#define fornrb(i, j, n) for (int i = n - 1; i >= j; i--)
#define fornb(i, n) for (int i = n - 1; i >= 0; i--)
#define forn1b(i, n) for (i = n; i > 0; i--)
#define fornn(i, n) for (int i = 1; i <= n; i++)
#define rety cout << "YES\n"; return 0;
#define retn cout << "NO\n"; return 0;
#define conty cout << "YES\n"; continue;
#define contn cout << "NO\n"; continue;
#define all(x) x.begin(), x.end()
#define mp make_pair
#define pb push_back
const int IMX = 2e9 + 5;
const ll LMX = 4e18 + 10, MOD = 1e9 + 7, MOD2 = 998244353;

const int MAXN = 2e5 + 5;

int n, xy[MAXN][2];
bool used[MAXN]; // edge
bool visited[MAXN][2]; // node
vector<int> adj[MAXN][2], id[MAXN][2], ans1, ans2;

void dfs(int p, bool type, int f = MAXN - 1) {
    visited[p][type] = 1;
    int q = -1, fid = -1;
    forn(i, adj[p][type].size()) {
        int z = adj[p][type][i], cid = id[p][type][i];
        if (z == f) {
            fid = cid;
            continue;
        }
        if (!visited[z][!type]) dfs(z, !type, p);
        if (!used[cid]) {
            if (q == -1) {
                q = cid;
            } else {
                ans1.pb(q);
                ans2.pb(cid);
                used[q] = 1;
                used[cid] = 1;
                q = -1;
            }
        }
    }
    if (q != -1 && f != MAXN - 1) {
        used[fid] = 1;
        used[q] = 1;
        ans1.pb(fid);
        ans2.pb(q);
    }
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int t;
    cin >> t;
    while(t--) {
        cin >> n;
        n *= 2;
        set<int> xs, ys;
        vector<int> xv, yv;
        forn(i, n) {
            cin >> xy[i][0] >> xy[i][1];
            if (xs.insert(xy[i][0]).second) xv.pb(xy[i][0]);
            if (ys.insert(xy[i][1]).second) yv.pb(xy[i][1]);
            adj[i][0].clear();
            adj[i][1].clear();
            id[i][0].clear();
            id[i][1].clear();
            visited[i][0] = 0;
            visited[i][1] = 0;
            used[i + 1] = 0;
        }
        ans1.clear();
        ans2.clear();
        sort(xv.begin(), xv.end());
        sort(yv.begin(), yv.end());
        int xm = xv.size(), ym = yv.size();
        forn(i, n) {
            int l = 0, r = xm - 1;
            while (l < r) {
                int m = (l + r) / 2;
                if (xv[m] == xy[i][0]) {
                    l = m;
                    r = m;
                } else if (xv[m] < xy[i][0]) l = m + 1;
                else r = m - 1;
            }
            xy[i][0] = l;
            l = 0, r = ym - 1;
            while (l < r) {
                int m = (l + r) / 2;
                if (yv[m] == xy[i][1]) {
                    l = m;
                    r = m;
                } else if (yv[m] < xy[i][1]) l = m + 1;
                else r = m - 1;
            }
            xy[i][1] = l;
            adj[xy[i][0]][0].pb(xy[i][1]);
            adj[xy[i][1]][1].pb(xy[i][0]);
            id[xy[i][0]][0].pb(i + 1);
            id[xy[i][1]][1].pb(i + 1);
        }
        forn(i, n) {
            if (!visited[i][0]) dfs(i, 0);
        }
        forn(i, n) {
            if (!visited[i][1]) dfs(i, 1);
        }
        cout << ans1.size() el;
        forn(i, ans1.size()) cout << ans1[i] sp ans2[i] el;
        vector<int> ans3;
        forn(i, n) if (!used[i + 1]) ans3.pb(i + 1);
        forn(i, ans3.size()) {
            cout << ans3[i] sp ans3[i + 1] el;
            i++;
        }
    }
}