const int maxn=2e5+100;
map<char,int>mp;
int get_nex(int x){
  for(int i=x+1;i<26;i++){
    char t=char(i+'a');
    if(mp[t]) return i;
  }
  return -1;
}
int main(){
  int _=read;
  while(_--){
    int n=read,k=read;
    string s;cin>>s;
    mp.clear();
    char minn='z';
    for(int i=0;s[i];i++) mp[s[i]]++,minn=min(minn,s[i]);
    //for(auto t:mp) cout<<t.first<<" "<<t.second<<endl;
    if(k>n){
      cout<<s;
      for(int i=0;i<k-n;i++) cout<<minn;
      puts("");
    }
    else{
      string t;
      for(int i=k-1;k>=0;i--){
        //cout<<s[i]-'a'<<endl;
        int tmp=get_nex(s[i]-'a');
        //cout<<tmp<<endl;
        if(tmp!=-1){
          t=char(tmp+'a')+t;
          t=s.substr(0,i)+t;
          break;
        }
        t=minn+t;
      }
      cout<<t<<endl;
    }
  }
  return 0;
}