#include <iostream>
#include <vector>
#include <map>
#include <string.h>
#include <math.h>
#include <set>
#include <stack>
#include <queue>
#include <bits/stdc++.h>

using namespace std;

#define ll long long
#define cy cout << "YES\n"
#define cn cout << "NO\n"
#define nl "\n"
#define fi first
#define se second
#define MOD 1000000007
#define all(v) v.begin(), v.end()
#define rall(v) v.end(), v.begin()

#define sz(s) s.size()
#define f0r(j, n) for (ll i = j; i < n; i++)
template <typename T = int>
istream &operator>>(istream &in, vector<T> &v)
{
    for (auto &x : v)
        in >> x;
    return in;
}

template <typename T = int>
ostream &operator<<(ostream &out, const vector<T> &v)
{
    for (const T &x : v)
        out << x << " ";
    return out;
}

void sherry()
{
    ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout);
#endif
}
/*
int gcd(ll a, ll b)
{
    if (b == 0)
        return a;
    return (b, a % b);
}
*/

void solve()
{
    int n, m;
    cin >> n >> m;
    string s;
    bool flag = 0;
    cin >> s;
    for (int i = 0; i < sz(s); i++)
    {
        if (s[i] - '0' < m and flag == 0)
        {
            cout << m;
            flag = 1;
        }
        cout << s[i];
    }
    if (flag == 0)
        cout << m;
    cout << nl;
}
int main()
{
    sherry();
    int t = 1;
    cin >> t;
    while (t--)
        solve();
    return 0;
}
//