#include <stdio.h>
#include <string.h>
#define NMAX 1011
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, p, q, md;
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]=='F')
S1[i-1] = 'F';
if (nq==0) {
x = 0;
if (A[i][j]>=K) t = 1;
else t = 0;
}
else
{
x = nq; t = 0;
p = 0; q = nq-1;
while (p<=q)
{
md = (p+q)>>1;
if ((T[md]<j-1)&&(A[i][j]+md >= K)) q = md-1;
else p = md+1;
}
x = q;
if (x<0) x = 0;
if ( (T[x] >= j-1)||(A[i][j]+x < K) ) x++;
if ( (x>0)&&(T[x-1] < j-1)&&(A[i][j]+x >= K) ) x--;
//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;
}