#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
// #define cerr cout
#include "algo/debug.h"
#else
#define deb(...) 30
#endif
#define ll long long
#define all(x) (x).begin(), (x).end()
#define f(i, n) for (int i = 0; i < n; i++)
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int t;
cin >> t;
while (t--)
{
int n, k;
cin >> n >> k;
vector<string> v(n);
for (auto &a : v)
cin >> a;
// vector<string> final;
string final = "";
for (int i = 0; i <= n - k - 1; i++)
{
string temp = "";
for (int j = 0; j <= k; j++)
{
temp += v[i + j];
}
// cout << ".\n";
if (temp > final)
final = temp;
}
int f = 1;
for (int i = 0; i < final.size(); i++)
{
if (final[i] == '0' and f)
continue;
else if (final[i] != '0')
f = 0, cout << final[i];
else if (!f)
cout << '0';
}
if (f)
cout << "0\n";
// cout << final << '\n';
}
return 0;
}