#pragma GCC optimize("Ofast","unroll-loops","omit-frame-pointer","inline")
#pragma GCC option("arch=native","tune=native","no-zero-upper")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2")
#pragma GCC optimize(3)
#include<bits/stdc++.h>
#define N 200010
#define inf 0x3f3f3f3f
#define rep(i,l,r) for(int i=l;i<=r;i++)
#define drep(i,r,l) for(int i=r;i>=l;i--)
using namespace std;
int rd() {
	int res=0,f=1;char ch=getchar();
	while(ch<'0'||ch>'9') {if(ch=='-') f*=-1;ch=getchar();}
	while(ch>='0'&&ch<='9') res=(res<<1)+(res<<3)+(ch^48),ch=getchar();
	return res*f;
}
int T,n,m,B,dis[N],st[N],top;
vector<int>G[N];
queue<int>qu;
void bfs(int s) {
	qu.push(s),dis[s]=0;
	int x;
	while(!qu.empty()) {
		x=qu.front(),qu.pop();
		for(auto v:G[x]) {
			if(dis[v]>dis[x]+1&&dis[x]+1<=B) qu.push(v),dis[v]=dis[x]+1;
		}
	}
}
int main() {
	T=rd();
	while(T--) {
		n=rd(),m=rd();
		B=sqrt(n);if(B*B<n)B++;
		rep(i,1,n) G[i].clear();
		for(int i=1,u,v;i<=m;i++) {
			u=rd(),v=rd();
			G[u].push_back(v),G[v].push_back(u);
		}
		rep(i,1,n) dis[i]=inf;
		rep(i,1,n) if(dis[i]==inf&&top<=B) bfs(i),st[++top]=i;
		if(top<=B) {
			printf("%d\n",top);
			while(top) printf("%d ",st[top]),top--;
			puts("");
		}
		else puts("-1");
	}
	return 0;
}