/*
 * AGGRCOW.cpp
 *
 *  Created on: May 29, 2014
 *      Author: Arpit Bhayani
 */

#include <cstdio>
#include <cstdlib>
#include <iostream>

using namespace std;

#define DEBUG 0
#define gc getchar

void scanint(int &x) {
    register int c = gc();
    x = 0;
    for (; (c < 48 || c > 57); c = gc())
        ;
    for (; c > 47 && c < 58; c = gc()) {
        x = (x << 1) + (x << 3) + c - 48;
    }
}

int compar(const void * a, const void * b) {
    return (*(int *) a) - (*(int *) b);
}

int can_adjust(int * array, int n, int k, int x) {

    int c = 1;
    int last = array[0];

    for (int i = 1; i < n; i++) {
        if (array[i] - last >= x) {
            c++;

            last = array[i];

            if (c == k)
                return 1;
        }
    }

    return 0;
}

int search(int * array, int n, int k) {

    int low = 0;
    int high = array[n - 1];

    while (low < high) {
        int mid = low + (high - low) / 2;

        if (can_adjust(array, n, k, mid) == 1) {
            low = mid + 1;
        } else {
            high = mid;
        }
    }

    return low - 1;
}

int main(int argc, char * argv[]) {

    int t;
    scanf("%d", &t);
    while (t--) {
        int n, k;
        scanint(n);
        scanint(k);
        int array[n];

        for (int i = 0; i < n; i++) {
            scanint(array[i]);
        }

        qsort(array, n, sizeof(int), compar);

        cout << search(array, n, k) << endl;

    }
    return 0;
}