#include <bits/stdc++.h>
#define rep(i,j,k) for(int i = (j);i <= (k);i++)
#define per(i,j,k) for(int i = (j);i >= (k);i--)
#define mod 998244353
#define SZ(S) (int)S.size()
#define eb emplace_back
using namespace std;

inline void add(int &x,int y){
	x += y;
	if(x >= mod) x -= mod;
}

inline void sub(int &x,int y){
	x -= y;
	if(x < 0) x += mod;
}
int T,n;
int pu[200005],pv[200005],ans[200005];

int m;
map <int,int> I[2];

int fa[400005];
int fnd(int u){
	if(fa[u] == u) return u;
	return fa[u] = fnd(fa[u]);
}

int mrg(int u,int v){
	u = fnd(u);v = fnd(v);
	if(u == v) return 0;
	fa[v] = u;
	return 1;
}

int cnt;
int head[400005];
struct eg{
	int to,nxt,pos,op;
}edge[400005];
vector <int> S[400005];

void make(int u,int v,int pos,int op){
	edge[++cnt].to = v;
	edge[cnt].pos = pos;
	edge[cnt].op = op;
	edge[cnt].nxt = head[u];
	head[u] = cnt;
}
int vis[400005];
void dfs(int u,int f,int id){
	vis[u] = 1;
	for(int i = head[u];i;i = edge[i].nxt){
		int v = edge[i].to;
		if(v == f) continue;
		dfs(v,u,i);
	}
	if(!f) return;
	if(SZ(S[u]) % 2){
		ans[edge[id].pos] = edge[id].op ^ 1;
		S[u].eb(edge[id].pos);
	}else{
		ans[edge[id].pos] = edge[id].op;
		S[f].eb(edge[id].pos);
	}
}

void solve(){
	scanf("%d",&n);
	cnt = 0;
	fill(head + 1,head + 4 * n + 1,0);
	rep(i,1,4 * n){
		fa[i] = i;
		S[i].clear();
		vis[i] = 0;
	}
	I[0].clear();I[1].clear();
	m = 0;
	int x,y;
	rep(i,1,2 * n){
		scanf("%d%d",&x,&y);
		if(!I[0][x]) I[0][x] = ++m;
		pu[i] = I[0][x];
		if(!I[1][y]) I[1][y] = ++m;
		pv[i] = I[1][y];
		if(mrg(pu[i],pv[i])){
			make(pu[i],pv[i],i,0);
			make(pv[i],pu[i],i,1);
		}else{
			ans[i] = 0;
			S[pu[i]].eb(i);
		}
	}
	rep(i,1,4 * n) if(!vis[i]) dfs(i,0,0);

	int answer = 0;
	rep(i,1,4 * n) answer += SZ(S[i]) / 2;
/*	rep(i,1,4 * n){
		if(S[i].empty()) continue;
		printf("Set %d\n",i);
		for(int u:S[i]) printf("%d ",u);
		printf("\n");
	}*/
	printf("%d\n",answer);
	int ext = -1;
	rep(i,1,4 * n){
		while(SZ(S[i]) >= 2){
			x = S[i].back();S[i].pop_back();
			y = S[i].back();S[i].pop_back();
			printf("%d %d\n",x,y);
		}
		if(S[i].empty()) continue;

		if(ext != -1){
			printf("%d %d\n",ext,S[i].back());
			ext = -1;
		}else{
			ext = S[i].back();
		}
	}
}

int main(){
//	freopen("test.in","r",stdin);
	scanf("%d",&T);
	while(T--) solve();
	return 0;
}
//g++ H.cpp -o H -Wall -Wshadow -O2 -std=c++14 -Wno-unused-result