You are an expert competitive programmer. You will be given a problem statement, test case constraints and example test inputs and outputs. Please reason step by step about the solution (that must respect memory and time limits), then provide a complete implementation in c++17.

Your solution must read input from standard input (cin), write output to standard output (cout).
Do not include any debug prints or additional output.

Put your final solution within a single code block:
```cpp
<your code here>
```

Execution time limit: 2.0 seconds
Memory limit: 256.0 MB

# Problem
Phoenix has a string $$$s$$$ consisting of lowercase Latin letters. He wants to distribute all the letters of his string into $$$k$$$ non-empty strings $$$a_1, a_2, \dots, a_k$$$ such that every letter of $$$s$$$ goes to exactly one of the strings $$$a_i$$$. The strings $$$a_i$$$ do not need to be substrings of $$$s$$$. Phoenix can distribute letters of $$$s$$$ and rearrange the letters within each string $$$a_i$$$ however he wants.

For example, if $$$s = $$$ baba and $$$k=2$$$, Phoenix may distribute the letters of his string in many ways, such as:

- ba and ba
- a and abb
- ab and ab
- aa and bb

But these ways are invalid:

- baa and ba
- b and ba
- baba and empty string ($$$a_i$$$ should be non-empty)

Phoenix wants to distribute the letters of his string $$$s$$$ into $$$k$$$ strings $$$a_1, a_2, \dots, a_k$$$ to minimize the lexicographically maximum string among them, i. e. minimize $$$max(a_1, a_2, \dots, a_k)$$$. Help him find the optimal distribution and print the minimal possible value of $$$max(a_1, a_2, \dots, a_k)$$$.

String $$$x$$$ is lexicographically less than string $$$y$$$ if either $$$x$$$ is a prefix of $$$y$$$ and $$$x \ne y$$$, or there exists an index $$$i$$$ ($$$1 \le i \le min(|x|, |y|))$$$ such that $$$x_i$$$ < $$$y_i$$$ and for every $$$j$$$ $$$(1 \le j < i)$$$ $$$x_j = y_j$$$. Here $$$|x|$$$ denotes the length of the string $$$x$$$.

## Input Format
The input consists of multiple test cases. The first line contains an integer $$$t$$$ ($$$1 \le t \le 1000$$$) — the number of test cases. Each test case consists of two lines.

The first line of each test case consists of two integers $$$n$$$ and $$$k$$$ ($$$1 \le k \le n \le 10^5$$$) — the length of string $$$s$$$ and the number of non-empty strings, into which Phoenix wants to distribute letters of $$$s$$$, respectively.

The second line of each test case contains a string $$$s$$$ of length $$$n$$$ consisting only of lowercase Latin letters.

It is guaranteed that the sum of $$$n$$$ over all test cases is $$$\le 10^5$$$.

## Output Format
Print $$$t$$$ answers — one per test case. The $$$i$$$-th answer should be the minimal possible value of $$$max(a_1, a_2, \dots, a_k)$$$ in the $$$i$$$-th test case.

## Examples
```input
6
4 2
baba
5 2
baacb
5 3
baacb
5 3
aaaaa
6 4
aaxxzz
7 1
phoenix
```
```output
ab
abbc
b
aa
x
ehinopx
```

## Note
In the first test case, one optimal solution is to distribute baba into ab and ab.

In the second test case, one optimal solution is to distribute baacb into abbc and a.

In the third test case, one optimal solution is to distribute baacb into ac, ab, and b.

In the fourth test case, one optimal solution is to distribute aaaaa into aa, aa, and a.

In the fifth test case, one optimal solution is to distribute aaxxzz into az, az, x, and x.

In the sixth test case, one optimal solution is to distribute phoenix into ehinopx.

Now solve the problem and return the code.