#include "bits/stdc++.h"
using namespace std;
#define ll long long int

const int M = 1e9 + 7;

int main() {
  int t;
  cin >> t;
  while (t--) {
    int n, x;
    cin >> n >> x;
    string s;
    cin >> s;
    int req = n; // changed here

    if (x == 0) {
      s.insert(n, "0");  // changed here
      cout << s << endl;
    }
    //int p = stoi(s[i])
    else {
      vector<int> digits;

      for (auto x : s)
        digits.push_back(x - '0');

      for (int i = 0; i < n; ++i) {
        if (digits[i] < x) {
          req = i;
          break;
        }
      }

      string y;
      y = to_string(x);

      s.insert(req, y);
      cout << s << endl;
    }
  }
  return 0;

}