#include <bits/stdc++.h>
using namespace std;

#define pb push_back
#define eb emplace_back
#define mk make_pair
#define fi first
#define sec second
#define fori(i, a, b) for(int i = int(a); i < int(b); i++)
#define cc(x)   cout << #x << " = " << x << endl
#define ok      cout << "ok" << endl

typedef pair<int,int> ii;
typedef long long ll;
const int INF = 0x3f3f3f3f;
const double PI = acos(-1.0);

const int N = 1e5 + 10;
ll n, c, cows[N];

bool check(int x, int c, int n){
    int first = cows[0];
    int cont = 1;

    for(int i = 1; i < n; i++){
        if(cows[i] - first >= x){
            first = cows[i];
            cont++;

            if(cont == c) return true;
        }
    }
    return false;
}

int busca(int ini, int fim, int c, int n){
    while(ini <= fim){
        int mid = (ini+fim)/2;

        if(check(mid, c, n)) ini = mid+1;
        else fim = mid-1;
    }
    return ini-1;
}

int main(){
    ios_base::sync_with_stdio(false);

    int tc; cin >>tc;
    while(tc--){
        cin >>n >>c;

        for(int i = 0; i < n; i++)
            cin >>cows[i];

        sort(cows, cows+n);
        cout <<busca(cows[0], cows[n-1], c, n) <<endl;
    }
    return 0;
}