AString

#include <bits/stdc++.h>
using namespace std;
#define PB push_back
#define ZERO (1e-10)
#define INF (1<<29)
#define CL(A,I) (memset(A,I,sizeof(A)))
#define DEB printf("DEB!\n");
#define D(X) cout<<"  "<<#X": "<<X<<endl;
#define EQ(A,B) (A+ZERO>B&&A-ZERO<B)
typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef pair<int,int> ii;
typedef vector<ii> vii;
#define IN(n) int n;scanf("%d",&n);
#define FOR(i, m, n) for (int i(m); i < n; i++)
#define REP(i, n) FOR(i, 0, n)
#define F(n) REP(i, n)
#define FF(n) REP(j, n)
#define FT(m, n) FOR(k, m, n)
#define aa first
#define bb second
void ga(int N,int *A){F(N)scanf("%d",A+i);}
#define MX (1<<17)
#define pc(C) putchar_unlocked(C)
void pp(char*s,int xx[MX][26],int L){
    F(26)xx[L][i]=-1;
    for(int i(L-1);~i;--i){
        FF(26)xx[i][j]=xx[i+1][j];
        xx[i][s[i]-97]=i;
    }
}
char s[MX];
int N,X[MX][26],K,I;
int main(void){
    IN(tt)F(tt){
        scanf("%s%d",s,&K),N=strlen(s),pp(s,X,N),I=0;
        while(K)F(26)if(~X[I][i]&&N-X[I][i]>=K)pc(i+97),I=X[I][i]+1,i=666,--K;
        pc(10);
    }
    return 0;
}