#include <bits/stdc++.h>
#define all(x) (x).begin(), (x).end()
using namespace std;
typedef long long ll;
int main() {
    ios::sync_with_stdio(0); cin.tie(0);
    int z; cin >> z;
    while (z--) {
        int n; char K; cin >> n >> K;
        string s, t; cin >> s >> t;
        bool good = false;
        for (int i = 1; i <= 26; i++) {
            int idx = n - 1;
            s[idx]++;
            while (idx >= 0 && s[idx] > 'z') {
                s[idx] = 'a';
                idx--;
                s[idx]++;
            }
            if (idx < 0 || s >= t) break;
            bool found = false;
            for (auto &x: s) {
                if (x == K) found = true;
            }
            if (found) {
                good = true;
                cout << s << '\n'; break;
            }
        }
        if (!good) cout << "NO\n";
    }
    return 0;
}