using namespace std;
#include <stdio.h>
#include <string.h>

#define alfa 26
#define Lmax 1024
#define infinit 0x3f3f3f3f

int T,K,N,M;
int a[Lmax][Lmax];
int p1[Lmax][alfa+1],p2[Lmax][alfa+1];
char s1[Lmax],s2[Lmax];

inline int maxim(int a,int b) { return a>b?a:b; }

void solve()
{
 int i,j,x,y,lx,ly;

 for (i=0;i<=N;++i)
     a[i][M]=0;
 for (i=0;i<=M;++i)
     a[N][i]=0;
 for (i=N-1;i>=0;--i)
     for (j=M-1;j>=0;--j)
         if (s1[i]==s2[j]) a[i][j]=a[i+1][j+1]+1;
            else a[i][j]=maxim(a[i][j+1],a[i+1][j]);

 memset(p1[N],0x3f,sizeof(p1[N]));
 memset(p2[M],0x3f,sizeof(p2[M]));
 for (i=N-1;i>=0;--i)
     {
      memcpy(p1[i],p1[i+1],sizeof(p1[i+1]));
      p1[i][s1[i]-'A']=i;
     }
 for (i=M-1;i>=0;--i)
     {
      memcpy(p2[i],p2[i+1],sizeof(p2[i+1]));
      p2[i][s2[i]-'A']=i;
     }

 lx=ly=0;
 for (i=1;i<=K;++i)
     {
      for (j=alfa;j>=0;--j)
          {
           x=p1[lx][j];
           y=p2[ly][j];
           if (x+y>=infinit) continue;
           if (a[x+1][y+1]+i>=K)
              {
               lx=x+1;
               ly=y+1;
               printf("%c",j+'A');
               break;
              }
          }
     }
 while (x+y<infinit)
       {
        for (j=alfa;j>=0;--j)
            {
             x=p1[lx][j];
             y=p2[ly][j];
             if (x+y>=infinit) continue;
             lx=x+1;
             ly=y+1;
             printf("%c",j+'A');
            }
       }
 printf("\n");
}

int main()
{
 freopen("seif.in","r",stdin);
 freopen("seif.out","w",stdout);
 scanf("%d\n",&T);
 while (T)
       {
        --T;
        scanf("%d\n",&K);
        scanf("%s",&s1);
        N=strlen(s1);
        scanf("%s",&s2);
        M=strlen(s2);
        solve();
       }
 fclose(stdin);
 fclose(stdout);
 return 0;
}