#include<bits/stdc++.h>
#define int long long

using namespace std;

signed main(){
    ios::sync_with_stdio(0); cin.tie(0);
//    freopen("input.txt","r",stdin);
    //freopen("output.txt","w",stdout);

    int t;

    cin >> t;

    while (t--) {
        int n, k;

        cin >> n >> k;

        vector<string> a(n);

        int maxLen = 0;

        for (string &u: a) {
            cin >> u;
            maxLen = max(maxLen, (int) u.size());
        }

        string res;



        for (int i = 0 ; i < maxLen ; i++) {
            vector<int> cnt(26, 0);

            for (string &u: a) {
                if (i < u.size()) {
                    cnt[u[i]-'a'] += 1;
                }
            }

//            if (i == 1)
//            cout << cnt['s' - 'a'] << endl;

            int cc = 0;

            for (int j = 0 ; j < 26 ; j++) {
                if (cnt[j] > 0) {
                    cc += 1;
                }
            }

            if (cc >= min((int) a.size(), k)) {
                break;
            }

            vector<string> b;

            for (int j = 0 ; j < 26 ; j++) {
                if (cnt[j] == 0) continue;
                if (cc - 1 + cnt[j] >= min(k, (int) a.size())) {
                    res.push_back(char(j + 'a'));
                    for (string &u: a) {
                        if (i < u.size() && u[i] - 'a' == j) {
                            b.push_back(u);
                        }
                    }
                    break;
                }
            }

            a = b;
        }

        if (res.size() == 0) {
            res = "EMPTY";
        }

        cout << res << endl;
    }
    return 0;
}