#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define endl '\n'
void solve()
{
int n,k; cin >> n >> k;
string arr[n];
for (int i = 0; i < n; i++) {
cin >> arr[i];
}
string ans = "0";
for (int i = 0; i < n-k; i++) {
string s = arr[i]+arr[i+1]+arr[i+2];
//cout << s << endl;
if (ans.size() < s.size()) {
ans = s;
}
else if (ans.size() == s.size()) {
for (int j = 0; j < ans.size(); j++) {
if (s[j] > ans[j]) {
ans = s;
break;
}
else if (ans[j] > s[j]) {
break;
}
}
}
}
cout << ans << endl;
}
int32_t main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
int t = 1; cin >> t;
while(t--) solve();
return 0;
}