#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long t, n;
  cin >> t;
  while (t--) {
    cin >> n;
    char a[2 * n - 1], b[n];
    long long i;
    for (i = 0; i < 2 * n - 1; i++) {
      cin >> a[i];
    }
    long long top = 0;
    while (top < n) {
      long long first = top;
      cout << a[first + top];
      top++;
    }
    cout << "\n";
  }
  return 0;
}
