#include <bits/stdc++.h>

#define F first
#define S second
#define pb push_back
#define all(x) x.begin(), x.end()
#pragma GCC optimise ("ofast")
#pragma GCC optimise("unroll-loops")

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

const int N = 200000 + 10;
const ll MOD = 1000000000 + 7;
const ll INF = 1000000000000000000;
const ll LOG = 25;
const int H = 313;

int Main(){
	string s, t;
	int n;
	char k;
	cin >> n >> k >> s >> t;
	int koj = -1;
	bool f = 0;
	for (int i = 0; i < n - 1; i++){
		if (t[i] == s[i] && s[i] == k) f = 1;
		if (t[i] != s[i] && koj == -1) koj = i;
		if (t[i] - s[i] > 1){
			for (int j = 0; j < i; j++) cout << s[j];
			cout << char(s[i] + 1);
			for (int j = i + 1; j < n; j++) cout << k;
			cout << '\n';
			return 0;
		}
		if (koj != -1) break;
	}
	if (koj == -1){
		for (int i = s[n - 1] - 'a' + 1; i < t[n - 1] - 'a'; i++){
			if (i == k - 'a' || f){
				for (int j = 0; j < n - 1; j ++) cout << s[j];
				cout << char(i + 'a');
				cout << '\n';
				return 0;
			}
		}
		return cout << "NO\n", 0;
	}
	for (int i = s[n - 1] - 'a' + 1; i < 26; i++){
		if (i == k - 'a' || f || s[koj] == k){
			for (int j = 0; j < n - 1; j++) cout << s[j];
			cout << char(i + 'a') << '\n';
			return 0;
		}
	}
	
	for (int i = n - 2 ; i > koj; i--){
		if (s[i] != 'z'){
			for (int j = 0; j < i; j++) cout << s[j];
			cout << 'z';
			for (int j = i + 1; j < n; j++) cout << k;
			cout << '\n';
			return 0;
		}
	}
	for (int i = 0; i < t[n - 1] - 'a'; i++){
		if (i == k - 'a' || f || t[koj] == k){
			for (int j = 0; j < n - 1; j++) cout << t[j];
			cout << char(i + 'a') << '\n';
			return 0;
		}
	}
	for (int i = n - 2; i > koj; i--){
		if (t[i] != 'a'){
			for (int j = 0; j < i; j++) cout << t[j];
			cout << 'a';
			for (int j = i + 1; j < n; j++) cout << k;
			cout << '\n';
			return 0;
		}
	}
	cout << "NO\n";
	return 0;
}

int main(){
	ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
	int t;
	cin >> t;
	while (t--) Main();
	



}