#include<stdio.h>
#include<string.h>
#include<iostream>
#include<queue>
#include<string>
#define INF 0xfffffff
#define min(a,b) (a>b?b:a)
using namespace std;
int dfn[20005*3],low[20005*3],stack[20005*3],head[20005*3],ins[20005*3],belong[20005*3];
int cnt,top,time,n,taj,flag;
struct s
{
	int u,v,next;
}edge[50005*3];
void add(int u,int v)
{
	edge[cnt].u=u;
	edge[cnt].v=v;
	edge[cnt].next=head[u];
	head[u]=cnt++;
}
void init()
{
	memset(dfn,-1,sizeof(dfn));
	memset(stack,0,sizeof(stack));
	memset(low,-1,sizeof(low));
	memset(head,-1,sizeof(head));
	memset(ins,0,sizeof(ins));
	memset(belong,-1,sizeof(belong));
	cnt=top=time=taj=0;
}
void tarjan(int u)
{
	dfn[u]=low[u]=time++;
	ins[u]=1;
	stack[top++]=u;
	for(int i=head[u];i!=-1;i=edge[i].next)
	{
		int v=edge[i].v;
		if(dfn[v]==-1)
		{
			tarjan(v);
			low[u]=min(low[v],low[u]);
		}
		else
		{	
			if(ins[v])
			{
				low[u]=min(dfn[v],low[u]);
				if(dfn[v]!=low[v])
				{
					flag=1;
				}
			}
		}
	}
	if(low[u]==dfn[u])
	{
		int now;
		taj++;
		do
		{
			now=stack[--top];
			ins[now]=0;
			if(belong[now]!=-1)
				flag=1;
			belong[now]=taj;
		}while(now!=u);
	}
}
int main()
{
	int t;
	scanf("%d",&t);
	while(t--)
	{
		int a,b;
		scanf("%d",&n);
		init();
		while(scanf("%d%d",&a,&b),a||b)
		{
			add(a,b);
		}
		flag=0;
		for(int i=0;i<n;i++)
		{
			if(dfn[i]==-1)
				tarjan(i);
		}
		if(taj==1&&flag==0)
			printf("YES\n");
		else
			printf("NO\n");
	}
}