#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef double db;
typedef string str;

typedef pair<int,int> pi;
typedef pair<ll,ll> pl;
typedef pair<db,db> pd;

typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<db> vd;
typedef vector<str> vs;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<pd> vpd;

#define ft front()
#define bk back()
#define pf push_front
#define pb push_back
#define eb emplace_back
#define f first
#define s second
#define sz(x) (int)x.size()
#define all(x) begin(x), end(x)
#define FOR(i,a,b) for(int i=(a);i<=(b);i++)
#define FORi(i,a,b) for(int i=(a);i>=(b);i--)
#define FORZ(i,a) for(int i=0;i<(a);i++)
#define FORZi(i,a) for(int i=(a)-1;i>=0;i--)
#define trav(a,x) for (auto& a: x)
#define what_is(x) cout << #x << " is " << x << "\n";

#define printl(a) cout << a << "\n";
#define prints(a) cout << a << " ";
#define printall(x) FORZ(i,sz(x))prints(x[i])
#define nextl cout << "\n"

#define in(a) cin >> a;

inline void io_setup(){
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
}

int main(){
  io_setup();
  int t;
  in(t);
  while(t--){
    int n,k;
    vector<char> vec;
    cin >> n;
    cin >> k;
    // string s;
    // cin >> s;
    FORZ(i,n){
      char a;
      cin >> a;
      vec.pb(a);
    }
    sort(vec.begin(),vec.end());

    int lastIndex = vec.size()-1;
    char lastChar = vec.at(lastIndex);
    int i;
    for(i=lastIndex;i>=0;i--){
      if(vec.at(i)!=lastChar)break;
    }
    lastIndex = max(i+1,k);
    // what_is(lastIndex);
    if(lastIndex!=k)lastIndex=vec.size();
    priority_queue <string ,vector<string>, greater<string>> pq;
    for(int i=0;i<k;i++){
      string temp = "";
      temp += vec.at(i);
      pq.push(temp);
    }
    string temp = pq.top();
    for(int i=k;i<lastIndex;i++){
      temp += vec.at(i);
      // what_is(i);
      // string temp = pq.top();
      // temp += vec.at(i);
      // what_is(temp);
    }
    // what_is(temp);
    pq.pop();
    pq.push(temp);
    for(int i=lastIndex;i<vec.size();i++){
      string temp = pq.top();
      temp += vec.at(i);
      pq.pop();
      pq.push(temp);
    }
    string MAX;
    while(pq.empty()==false){
      MAX = pq.top();
      pq.pop();
    }
    printl(MAX);
  }
}

/*
  `-.    .-'
     :  :
  --:--:--
   :  :
.-'    `-.  ~LaKsHiTh_
*/