#include<cstdio>
#include<iostream>
#include<algorithm>
#include<vector>
#include<cstring>
#include<map>
using namespace std;

int t; 
int n, k; 

int main() {
	scanf("%d", &t);
	while(t--) {
		scanf("%d%d", &n, &k);
		map<char, vector<string> > mp;
		for(int i=1; i<=n; i++) {
			string t; cin>>t; //哈哈 shift 不知道该怎么取消同步捏。 
			mp[t[0]].push_back(t);
		}
		if(mp.size()>=k) {
			puts("EMPTY");
			continue; 
		}
		string a, b; char qwq;
		for(auto it = mp.begin(); it!=mp.end(); it++) {
			sort(it->second.begin(), it->second.end());
			if(it->second.size()<k) {
				if(it->second.size()>1) qwq = it->first;
				k-=it->second.size();
				continue;
			}
			if(k==1) {
				a = mp[qwq][mp[qwq].size()-2];
				b = mp[qwq][mp[qwq].size()-1];
			} else {
				a = it->second[k-2];
				b = it->second[k-1];
			}
			break; //233
		}
		int i;
		for(i=0; i<min(a.size(), b.size()); i++) {
			if(a[i]!=b[i]) {
				break;
			}
		} 
		cout<<a.substr(0, i)<<endl;
	}
	return 0;
}