#include<cstdio>
#include<string>
using namespace std;
#define MAX_N 1024
int T, N,M;
int bst[MAX_N][MAX_N];
char A[MAX_N], B[MAX_N];
int K;
int pi[MAX_N][32], pj[MAX_N][32];
inline int max(int a, int b) {return a>b?a:b; }
int main()
{
	freopen("seif.in","r",stdin);
	freopen("seif.out","w",stdout);
	int i,j,c,ok;
	scanf("%d",&T);
	while(T--)
	{
		scanf("%d\n",&K);
		memset(bst,0,sizeof(bst));
		scanf("%s\n",A+1);
		scanf("%s\n",B+1);
		
		N = strlen(A+1); M = strlen(B+1);
		for(i=N;i>0;--i)
			for(j=M;j>0;--j)
				bst[i][j] = A[i]==B[j]? bst[i+1][j+1] + 1: max(bst[i+1][j],bst[i][j+1]);
			
		memset(pi,-1,sizeof(pi)); memset(pj,-1,sizeof(pj));
		
		for(i = N; i>0; --i)
			for(c = 0; c<26;++c)
				if(A[i] == (c+'A')) pi[i][c] = i;
				else pi[i][c] = pi[i+1][c];

		for(i = M; i>0; --i)
			for(c = 0; c<26;++c)
				if(B[i] == (c+'A')) pj[i][c] = i;
				else pj[i][c] = pj[i+1][c];
			
		i = 1; j = 1; ok = 1;
		while(ok)
		{
			ok = 0;
			for(c=25;c>=0 && !ok;--c)
				if(pi[i][c] != -1 && pj[j][c] != -1 && bst[pi[i][c]][pj[j][c]] >= K)
				{
					printf("%c",c+'A');
					ok = 1;
					i = pi[i][c]+1;
					j = pj[j][c]+1;
				}
            K--;
		}
		printf("\n");
	}
	return 0;
}