#include <stdio.h>  
#include <string.h>  
#define NMAX 2011

int Case;  
int N, M, K, A[NMAX][NMAX], T[NMAX], ok[NMAX];
char S1[NMAX], S2[NMAX], Sir[NMAX], temp[NMAX], caux, saux;

int main()  
{  
     int i, j, nq, nt, l, c, t, x, y, sx, sy, min;
     
     freopen("seif.in", "r", stdin);  
     freopen("seif.out", "w", stdout);  
   
     scanf("%d", &Case);  
   
     while (Case--)  
     {
            memset(S1,'\0',sizeof(S1));
            memset(S2,'\0',sizeof(S2));
            scanf("%d", &K);  
            scanf(" ");
            gets(S1);
            scanf(" ");
            gets(S2);

            N = strlen(S1); M = strlen(S2);  
   
            for (i = 0; i < N; i++) temp[i] = S1[N-i-1];  
            for (i = 0; i < N; i++) S1[i] = temp[i];  
            for (i = 0; i < M; i++) temp[i] = S2[M-i-1];  
            for (i = 0; i < M; i++) S2[i] = temp[i];
   
            memset(A,0,sizeof(A));
            memset(T,0,sizeof(T));
            memset(temp,'\0',sizeof(temp));
            memset(Sir,'\0',sizeof(Sir));
   
            for (i = 1; i <= N; i++)  
             for (j = 1; j <= M; j++)  
                 if (S1[i-1]==S2[j-1]) A[i][j] = A[i-1][j-1]+1;
                 else  
                 {  
                     A[i][j] = A[i][j-1];  
                     if (A[i][j]<A[i-1][j]) A[i][j] = A[i-1][j];  
                 }  
   
            nq = x = t = y = 0;

            memset(ok,0,sizeof(ok));
            memset(T,0,sizeof(T));
            for (i = N; i > 0; i--)
               for (j = M; j > 0; j--)
               if (!ok[j-1] && S1[i-1]==S2[j-1])
               {
                   if (S1[i-1]=='Z')
                      S1[i-1] = 'Z';
                   
                   if (nq==0) {
                      x = 0;
                      if (A[i][j]>=K) t = 1;
                      else t = 0;
                      }
                   else
                   {
                       x = nq; t = 0;

                       while ( (x>0) && (T[x-1] < j-1) && (A[i][j]+x >= K) ) x--;

                       if (x>0&&T[x-1] < j-1) continue;

                       if (x>0&&Sir[x]<S1[i-1]) t = 1;

                       //if (x<nq) x++;

                       while ( (x>0) && (A[i][j]+x-1 >= K) && (Sir[x-1] < S1[i-1]) ) x--, t = 1;

                       if ( (x==0&&Sir[0]<S1[i-1])||( (x==nq)&&(T[x-1] > j-1) ) ) t = 1;
                       if (A[i][j]+x < K) t = 0;

                       if (t==1)
                          for (l = nq-1; l >= x; l--) ok[ T[l] ] = 0;
                   }
                   if (t)
                   {
                      Sir[x] = S1[i-1], T[x++] = j-1;
                      nq = x;
                      ok[j-1] = 1;
                      break;
                   }
               }

            for (l = 0; l < nq; l++)
            {  
                 printf("%c", Sir[l]);
                 Sir[l] = '\0';  
            }  
            printf("\n");  
     }  
   
     return 0;  
       
}