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

using ll = long long;
#define int ll
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
using pii = pair<int, int>;
using vi = vector<int>;
#define fi first
#define se second
#define pb push_back

const ll mod = 1000000007; // faster if const

ll modpow(ll b, ll e) {
	ll ans = 1;
	for (; e; b = b * b % mod, e /= 2)
		if (e & 1) ans = ans * b % mod;
	return ans;
}
int matInv(vector<vector<ll>>& A) {
	int n = sz(A); vi col(n);
	vector<vector<ll>> tmp(n, vector<ll>(n));
	rep(i,0,n) tmp[i][i] = 1, col[i] = i;

	rep(i,0,n) {
		int r = i, c = i;
		rep(j,i,n) rep(k,i,n) if (A[j][k]) {
			r = j; c = k; goto found;
		}
		return i;
found:
		A[i].swap(A[r]); tmp[i].swap(tmp[r]);
		rep(j,0,n) swap(A[j][i], A[j][c]), swap(tmp[j][i], tmp[j][c]);
		swap(col[i], col[c]);
		ll v = modpow(A[i][i], mod - 2);
		rep(j,i+1,n) {
			ll f = A[j][i] * v % mod;
			A[j][i] = 0;
			rep(k,i+1,n) A[j][k] = (A[j][k] - f*A[i][k]) % mod;
			rep(k,0,n) tmp[j][k] = (tmp[j][k] - f*tmp[i][k]) % mod;
		}
		rep(j,i+1,n) A[i][j] = A[i][j] * v % mod;
		rep(j,0,n) tmp[i][j] = tmp[i][j] * v % mod;
		A[i][i] = 1;
	}

	for (int i = n-1; i > 0; --i) rep(j,0,i) {
		ll v = A[j][i];
		rep(k,0,n) tmp[j][k] = (tmp[j][k] - v*tmp[i][k]) % mod;
	}

	rep(i,0,n) rep(j,0,n)
		A[col[i]][col[j]] = tmp[i][j] % mod + (tmp[i][j] < 0 ? mod : 0);
	return n;
}
vector<pii> generalMatching(int N, vector<pii>& ed) {
	vector<vector<ll>> mat(N, vector<ll>(N)), A;
	for (pii pa : ed) {
		int a = pa.first, b = pa.second, r = rand() % mod;
		mat[a][b] = r, mat[b][a] = (mod - r) % mod;
	}

	int r = matInv(A = mat), M = 2*N - r, fi, fj;
	assert(r % 2 == 0);

	if (M != N) do {
		mat.resize(M, vector<ll>(M));
		rep(i,0,N) {
			mat[i].resize(M);
			rep(j,N,M) {
				int r = rand() % mod;
				mat[i][j] = r, mat[j][i] = (mod - r) % mod;
			}
		}
	} while (matInv(A = mat) != M);

	vi has(M, 1); vector<pii> ret;
	rep(it,0,M/2) {
		rep(i,0,M) if (has[i])
			rep(j,i+1,M) if (A[i][j] && mat[i][j]) {
				fi = i; fj = j; goto done;
		} assert(0); done:
		if (fj < N) ret.emplace_back(fi, fj);
		has[fi] = has[fj] = 0;
		rep(sw,0,2) {
			ll a = modpow(A[fi][fj], mod-2);
			rep(i,0,M) if (has[i] && A[i][fj]) {
				ll b = A[i][fj] * a % mod;
				rep(j,0,M) A[i][j] = (A[i][j] - A[fi][j] * b) % mod;
			}
			swap(fi,fj);
		}
	}
	return ret;
}
#undef sz

int naive(int n, vector<pair<int, int> > p)
{
    vector<pii> g;
    for (int i=0; i<n; i++)
        for (int j=i+1; j<n; j++)
            if (p[i].fi == p[j].fi || p[i].se == p[j].se)
                g.pb({i, j});
    return generalMatching(n, g).size();
}

vector<vector<array<int, 3> > > g;
vector<vector<int> > g2;
vector<int> used;
vector<pair<int, int> > ansv;
vector<int> sz;
vector<int> cur;

void dfs(int v)
{
    if (used[v])
        return;
    used[v] = 1;
    cur.pb(v);

    for (auto [i, x, y] : g[v])
        if (!used[i])
        {
            dfs(i);
            if (sz[i]%2 == 1)
            {
//                cout<<"ae "<<x<<" "<<y<<"\n";
                sz[i]++;
                sz[v]++;

                g2[x].pb(y);
                g2[y].pb(x);
            }
        }
}

void dfs2(int v)
{
    if (used[v])
        return;
    used[v] = 1;
    if (g2[v].size()%2==1)
        cur.pb(v);

    for (auto i : g2[v])
        if (!used[i])
            dfs2(i);
}

int solve(int n, vector<pair<int, int> > p)
{
    map<int, vector<int> > mp;
    map<int, int> cmp;
    int m = 0;
    for (int i=0; i<n; i++)
    {
        mp[p[i].fi].pb(i);
        if (!cmp.count(p[i].se))
            cmp[p[i].se] = m++;
    }
    vector<vector<int> > mp2(m);
    sz.assign(m, 0);
    for (int i=0; i<n; i++)
    {
        mp2[cmp[p[i].se]].pb(i);
        sz[cmp[p[i].se]]++;
    }

    g.clear();
    g.resize(m);
    for (const auto& [x, v] : mp)
    {
//        cout<<x<<"->"<<v.size()<<"\n";
        for (int i=0; i+1<int(v.size()); i++)
        {
            g[cmp[p[v[i]].se]].pb({cmp[p[v[i+1]].se], v[i], v[i+1]});
            g[cmp[p[v[i+1]].se]].pb({cmp[p[v[i]].se], v[i], v[i+1]});
        }
    }

    used.assign(m, 0);
    int ans = 0;
    ansv.clear();

//    cout<<m<<"\n";
//    for (int i=0; i<m; i++)
//    {
//        cout<<sz[i]<<" -> ";
//        for (auto [j, x, y] : g[i])
//            cout<<j<<" ";
//        cout<<"\n";
//    }
    g2.clear();
    g2.resize(n);
    for (int i=0; i<m; i++)
        if (!used[i])
    {
        cur.clear();
        dfs(i);

        int sum = 0;
        for (int i : cur)
            sum += sz[i];
        ans += sum/2;
    }
    used.assign(n, 0);
    for (int i=0; i<n; i++)
        if (!used[i])
    {
        cur.clear();
        dfs2(i);

        if (cur.size()%2==1)
            exit(7);

//        cout<<"cur : ";
//        for (int i : cur)
//            cout<<i<<" ";
//        cout<<"\n";

        for (int i=0; i+1<cur.size(); i+=2)
            ansv.pb({cur[i], cur[i+1]});
    }
    used.assign(n, 0);
    for (auto [x, y] : ansv)
        used[x] = used[y] = 1;
    int gl = -1;
    vector<pair<int, int> > sad;
    for (int i=0; i<m; i++)
    {
        int l = -1;
        for (int j : mp2[i])
            if (!used[j])
        {
            if (l == -1)
                l = j;
            else
            {
                ansv.pb({l, j});
                l = -1;
            }
        }
        if (l!=-1)
        {
            if (gl==-1)
                gl = l;
            else
            {
                sad.pb({gl, l});
                gl = -1;
            }
        }
    }

//    for (int i=0; i<m; i++)
//    {
//        cout<<"mp2 : ";
//        for (int j : mp2[i])
//            cout<<j<<" ";
//        cout<<"\n";
//    }

    set<int> st;
    for (auto [x, y] : ansv)
    {
        if (p[x].fi != p[y].fi && p[x].se != p[y].se)
            exit(4);
        st.insert(x);
        st.insert(y);
    }
    if (st.size() != 2*ansv.size())
        exit(5);
    int A = ansv.size();
    for (auto [x, y] : sad)
        ansv.pb({x, y});
    if (ansv.size() != n/2)
        exit(9);

    return A;
}

void solve()
{
    int n;
    cin>>n;
    n*=2;
    vector<pair<int, int> > v(n);
    for (auto& [x, y] : v)
        cin>>x>>y;

    cout<<solve(n, v)<<"\n";
    for (auto [x, y] : ansv)
        cout<<x+1<<" "<<y+1<<"\n";
//    cout<<naive(n, v)<<"\n";
}

mt19937 rng(time(0));
signed main()
{
	cin.tie(0)->sync_with_stdio(0);
	cin.exceptions(cin.failbit);

//	while (true)
//    {
//        int n = 2*(rng()%5+1);
//        int x = (sqrt(n)+1)*(rng()%2+2);
//
//        set<pair<int, int> > st;
//        while (st.size()<n)
//        {
//            int a = rng()%x;
//            int b = rng()%x;
//            st.insert({a, b});
//        }
//        vector<pair<int, int> > v;
//        for (auto [x, y] : st)
//            v.pb({x, y});
//        auto A = naive(n, v);
//        auto B = solve(n, v);
//
//        if (A != B)
//        {
//            bool ok = true;
//            for (int i=0; i<5; i++)
//                if (naive(n, v) != A)
//                {
//                    ok = false;
//                }
//            if (!ok)
//                continue;
//            cout<<"\n";
//            cout<<A<<" "<<B<<"\n";
////            cout<<naive(n, v)<<naive(n, v)<<naive(n, v)<<naive(n, v)<<naive(n, v)<<naive(n, v)<<naive(n, v)<<"\n";
//            cout<<n<<"\n";
//            for (auto [x, y] : v)
//                cout<<x<<" "<<y<<"\n";
//            exit(0);
//        }
//        else
//        {
//            cout<<"@";
//        }
//    }


    int t;
    cin>>t;
    while (t--)
        solve();
}