#include <algorithm>
#include <iostream>
#include <cstring>
#include <cstdio>
#include <cmath>
#define FOR() ll le=e[u].size();for(ll i=0;i<le;i++)
#define QWQ cout<<"QwQ\n";
#define ll long long
#include <vector>
#include <queue>
#include <map>
#include <set>

typedef long double db;
using namespace std;
const ll N=801010;
const ll qwq=303030;
const ll inf=0x3f3f3f3f3f;
const db eps = 1e-8;

inline ll read() {
    ll sum = 0, ff = 1; char c = getchar();
    while(c<'0' || c>'9') { if(c=='-') ff = -1; c = getchar(); }
    while(c>='0'&&c<='9') { sum = sum * 10 + c - '0'; c = getchar(); }
    return sum * ff;
}

ll T;
ll n,m;
map <ll,ll> hang; ll cnt,tot;
map <ll,ll> lie;
map <ll,ll> b2d;
vector <ll> e[N];
ll vis[N];
ll fa[N];
vector <ll> d[N];
ll belong[N][2];
set <ll> fei[N];
ll dp[N],tim[N];
struct D{
    ll dep,id,ti;
};
inline bool operator < (D A,D B) { if(A.dep!=B.dep) return A.dep < B.dep; return A.ti > B.ti; }
priority_queue <D> q;
ll dao[N];

ll ans;
ll st1[N],st2[N];

void chushihua() {
    hang.clear(); lie.clear(); b2d.clear();
    for(ll i=1;i<=cnt;i++) e[i].clear(), d[i].clear(), fei[i].clear(), dp[i] = tim[i] = dao[i] = fa[i] = 0;
    for(ll i=1;i<=max(2*n,cnt);i++) vis[i] = belong[i][0] = belong[i][1] = 0;
    cnt = tot = 0;
    ans = 0;
}

void DFS(ll u) {
    tim[u] = ++tot;
    vis[u] = 1;
    for(ll v : e[u]) {
        if(v==fa[u]) continue;
        if(vis[v]) {
            ll x = u, y = v;
            if(x>y) swap(x,y);
            ll id = b2d[x*4*n+y];
            fei[u].insert(id);
            fei[v].insert(id);
        }
        else {
            fa[v] = u;
            d[u].push_back(v);
            dp[v] = dp[u] + 1;
            DFS(v);
        }
    }
}

inline ll qiu(ll x,ll y) {
    if(x>y) swap(x,y);
    return b2d[x*4*n+y];
}

void check(ll u) {
    if(fei[u].size()!=0) {
        ll ant;
        ll dian = *fei[u].begin();
        if(belong[dian][0]==u) ant = belong[dian][1];
        if(belong[dian][1]==u) ant = belong[dian][0];
        fei[u].clear(); fei[ant].clear();
        vis[u] = 0;
        fa[u] = ant;

        // cout<<"gua !!!!! "<<u<<" -> "<<ant<<endl;
        d[ant].push_back(u);
        dp[u] = dp[ant] + 1;
        q.push({dp[ant]+1,u,++tot});
    }
}

void chuli(ll u,ll v) {
    // cout<<"chuli ("<<u<<","<<v<<")\n";
    ll dian1 = qiu(u,fa[u]), dian2 = qiu(v,fa[v]);
    // cout<<"pei ("<<dian1<<","<<dian2<<")\n";
    st1[++ans] = dian1; st2[ans] = dian2;

    vis[u] = vis[v] = 1;

    check(u);
    check(v);
}

int main() {
    // freopen("data.in","r",stdin);
    // freopen("data2.in","w",stdout);
    ll x,y;
    T = read();

    // cout<<T<<"\n";

    while(T--) {
        chushihua();
        n = read();

        // cout<<n<<"\n";

        for(ll i=1;i<=2*n;i++) {
            x = read(); y = read();

            // cout<<x<<" "<<y<<endl;

            if(!hang[x]) hang[x] = ++cnt;
            if(!lie[y]) lie[y] = ++cnt;
            x = hang[x]; y = lie[y];
            e[x].push_back(y);
            e[y].push_back(x);
            // cout<<x<<" <-> "<<y<<endl;
            if(x>y) swap(x,y);
            b2d[x*4*n+y] = i;
            belong[i][0] = x;
            belong[i][1] = y;
        }
        for(ll i=1;i<=cnt;i++) {
            if(!vis[i]) DFS(i);
        }

        // for(ll i=1;i<=cnt;i++) {
        //     // cout<<"tim["<<i<<"] = "<<tim[i]<<endl;
        //     // cout<<i<<" : \nson : ";
        //     for(ll v : d[i]) {
        //         cout<<v<<" "<<i<<"\n";
        //     }
        //     // cout<<"\nfei : ";
        //     // for(ll v : fei[i]) {
        //     //     cout<<v<<" ";
        //     // }
        //     // cout<<"\n\n";
        // }

        for(ll u=1;u<=cnt;u++) {
            while(fei[u].size()>=2) {
                set<ll>::iterator it = fei[u].begin(), nxt = it;
                nxt++;
                ll dian1 = (*it), dian2 = (*nxt);
                // cout<<"xiao ("<<dian1<<","<<dian2<<")\n";
                st1[++ans] = dian1; st2[ans] = dian2;

                ll b1 = belong[dian1][0], b2 = belong[dian1][1];
                fei[b1].erase(fei[b1].find(dian1));
                fei[b2].erase(fei[b2].find(dian1));
                b1 = belong[dian2][0], b2 = belong[dian2][1];
                fei[b1].erase(fei[b1].find(dian2));
                fei[b2].erase(fei[b2].find(dian2));
            }

        }

        for(ll i=1;i<=cnt;i++) vis[i] = 0;
        for(ll i=1;i<=cnt;i++) q.push({dp[i],i,tim[i]});
        while(!q.empty()) {
            D now = q.top();
            ll u = q.top().id; q.pop();
            // cout<<"u = "<<u<<"\n";
            if(vis[u] || !fa[u] || dp[u]!=now.dep) continue;
            ll pa = fa[u], papa = fa[pa];

            ll xiong = 0;
            while(1) {
                if(dao[pa]==d[pa].size()) break;
                int nxt = d[pa][dao[pa]];
                if(nxt==u || vis[nxt] || dp[nxt]!=now.dep) { dao[pa]++; continue; }
                xiong = nxt;
                break;
            }

            if(xiong) {
                chuli(u, xiong);
            }
            else if(papa) {
                chuli(u, pa);
            }
        }

        for(ll i=1;i<=2*n;i++) vis[i] = 0;
        cout<<ans<<"\n";
        for(ll i=1;i<=ans;i++) cout<<st1[i]<<" "<<st2[i]<<"\n", vis[st1[i]] = vis[st2[i]] = 1;
        ll ji = 0;
        for(ll i=1;i<=2*n;i++) {
            if(vis[i]) continue;
            cout<<i;
            if(ji) cout<<"\n";
            else   cout<<" ";
            ji^=1;
        }
    }

    return 0;
}
/*

6
4
3 2
1 2
3 4
1 5
1 6
1 7
1 4
3 7
5
1 5
2 5
3 5
2 4
3 4
5 4
0 3
4 3
2 2
4 1
5
1 5
2 5
3 5
2 4
3 4
5 4
2 3
4 3
2 2
2 1
2
0 0
0 1
1 0
1 1
2
0 0
0 1
0 2
0 3
2
0 0
1 1
2 2
3 3


*/