#include <bits/stdc++.h>
using namespace std;

const int nmax = 1e6+5;
const int alpha = 26;

struct node {
    int next[alpha];
    int leaf_cnt;
    int used_cnt;
    int actual_cnt;
} trie[nmax];

int node_cnt = 0;

int make_node(){
    int index = node_cnt;
    auto &nd = trie[index];
    node_cnt++;

    memset(nd.next, -1, sizeof(nd.next));
    nd.leaf_cnt = 0;
    nd.used_cnt = 0;
    nd.actual_cnt = 0;

    return index;
}

void insert(string& s){
    int cur = 0;

    for(char c : s){
        int x = c-'a';
        if(trie[cur].next[x] == -1){
            trie[cur].next[x] = make_node();
        }

        cur = trie[cur].next[x];
        trie[cur].actual_cnt++;

//        cout<<cur<< " ";
    }
    trie[cur].leaf_cnt++;
//    cout<<endl;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(NULL);

    int tc;
    cin>>tc;

    for(int cs = 1; cs <= tc; cs++){
        int n, k;
        cin>>n>>k;

        node_cnt = 0;
        make_node();

        for(int i = 0; i<n; i++){
            string s;
            cin>>s;
            insert(s);
        }

        auto get_first_unused = [&](auto next) {
            for(int x = 0; x < alpha; x++){
                if(next[x] == -1) continue;
                if(trie[next[x]].used_cnt == 0)
                    return next[x];
            }
            return -1;
        };

        auto get_first_usable = [&](auto next) {
            for(int x = 0; x < alpha; x++){
                if(next[x] == -1) continue;
                if(trie[next[x]].used_cnt != trie[next[x]].actual_cnt)
                    return next[x];
            }
            return -1;
        };

        for(int i = 0; i<k; i++){
            int cur = 0;
            while(true){
                int nxt = get_first_unused(trie[cur].next);
//                cout<<nxt<<endl;
//                getchar();

                if(nxt == -1){
                    nxt = get_first_usable(trie[cur].next);
                }

//                cout<<"FOR "<<i<<" "<<cur<<endl;

                cur = nxt;

                trie[cur].used_cnt++;

                if(trie[cur].leaf_cnt > 0){
                    trie[cur].leaf_cnt--;
                    break;
                }
            }
        }

        string ans;
        int cur = 0;
        while(true){
            bool ok = false;
            for(int x = alpha-1; x >= 0; x--){
                int nxt = trie[cur].next[x];
                if(nxt == -1) continue;

                if(trie[nxt].used_cnt > 1){
                    ans.push_back('a' + x);
                    cur = nxt;
                    ok = true;
                    break;
                }
            }

            if(!ok)
                break;
        }

        if(ans.empty()) ans = "EMPTY";

        cout<<ans<<"\n";
    }

    return 0;
}

/*
2
5 3
gdcpc
gdcpcpcp
suasua
suas
sususua
3 3
a
b
c
*/