#include<bits/stdc++.h>
using namespace std;
#define int long long
bool is_big(string &a, string &b){
    if(a.size() > b.size()) return true;
    else if(a.size() < b.size()) return false;
    else return a > b;  // If size is same, then sort lexicographically
}
signed main(){
    int t;
    cin >> t;
    while(t--){
        int n, k;
        cin >> n >> k;
        string ans , tmp;
        string s[n];
        int sz[n];
        for(int i = 0; i < n; i++){
            cin >> s[i];
            //Max = max(Max, s[i]);
            if(is_big(s[i], ans)){
                ans = s[i];
            }
            tmp += s[i];
            int id = i - k - 1;
            sz[i] = s[i].size();
            if(id >= 0) tmp = tmp.substr(sz[id], tmp.size() - (sz[id]));
            if(is_big(tmp, ans)) ans = tmp;
           // ans = max(ans, tmp);
            // if(tmp.size() > ans.size()) ans = tmp;
            // else if( tmp.size() == ans.size() and tmp > ans) ans = tmp;
            
        }
        //if(k == 0) ans = Max;
        cout << ans << "\n";
    }
    return 0;
}