package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {
	reader := bufio.NewReader(os.Stdin)
	writer := bufio.NewWriter(os.Stdout)
	defer writer.Flush()

	var t int
	if _, err := fmt.Fscan(reader, &t); err != nil {
		return
	}

	for i := 0; i < t; i++ {
		var n, k int
		fmt.Fscan(reader, &n, &k)
		var s string
		fmt.Fscan(reader, &s)

		m := n / k
		cnt := make([]int, 26)
		for j := 0; j < len(s); j++ {
			cnt[s[j]-'a']++
		}

		ans := make([]byte, k)
		for j := 0; j < k; j++ {
			c := 0
			for c < m && c < 26 && cnt[c] > 0 {
				cnt[c]--
				c++
			}
			ans[j] = byte('a' + c)
		}
		fmt.Fprintln(writer, string(ans))
	}
}