//Author: Kevin
#include<bits/stdc++.h>
//#pragma GCC optimize("O2")
using namespace std;
#define ll long long
#define ull unsigned ll
#define pb emplace_back
#define mp make_pair
#define ALL(x) (x).begin(),(x).end()
#define rALL(x) (x).rbegin(),(x).rend()
#define srt(x) sort(ALL(x))
#define rev(x) reverse(ALL(x))
#define rsrt(x) sort(rALL(x))
#define sz(x) (int)(x.size())
#define inf 0x3f3f3f3f
#define pii pair<int,int>
#define lb(v,x) (int)(lower_bound(ALL(v),x)-v.begin())
#define ub(v,x) (int)(upper_bound(ALL(v),x)-v.begin())
#define uni(v) v.resize(unique(ALL(v))-v.begin())
#define longer __int128_t
void die(string S){puts(S.c_str());exit(0);}
void add(string &s)
{
	int p=sz(s)-1;
	s[p]++;
	while(p&&s[p]>'z')
	{
		s[p]='a';
		p--;
		s[p]++;
	}
}
void sub(string &s)
{
	int p=sz(s)-1;
	s[p]--;
	while(p&&s[p]<'a')
	{
		s[p]='z';
		p--;
		s[p]--;
	}
}
int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	int t;
	cin>>t;
	while(t--)
	{
		int n;
		char ch;
		cin>>n>>ch;
		string s,t;
		cin>>s>>t;
		add(s);
		bool fd=0;
		while(s<t)
		{
			bool ok=0;
			for(auto c:s)
				if(c==ch)
					ok=1;
			if(ok)
			{
				cout<<s<<'\n';
				fd=1;
				break;
			}
			add(s);
		}
		if(!fd)
			cout<<"NO\n";
	}
	return 0;
}