//TheShadowGrapher's Code
#include <iostream>
#include<bits/stdc++.h>
#include <queue>
#include <vector>
#include <set>
#include <map>
#define f(i,a,b,c) for(int i = a ; i< b ; i+=c )
#define nl "\n"
using namespace std;
bool checkIf(int key , int a[] , int nstalls , int ncows)
{
int cs = 0 , i = 1 ,count = 1 ;
while(i < nstalls )
{
if(abs(a[i]-a[cs]) >= key){ cs = i ; count++ ; }
i++ ;
if(count >=ncows ) return true ;
}
return false ;
}
void anything()
{
int n ,c ; cin>>n ; cin>>c ; //inputs
int a[n] ;
int MinDist = 1000001 , maxVal = -1 , minVal = 100000001 , temp;
f(i, 0 , n , 1)
{
cin>>a[i];
if (a[i] > maxVal ) maxVal = a[i] ;
if (a[i] <minVal) minVal = a[i] ;
if(i)
{
if(temp-a[i] < MinDist) MinDist = temp-a[i] ;
temp = a[i];
}
else temp = a[i] ;
}
sort(a, a+n) ;
int low = MinDist , high = maxVal-minVal , ans = 0 ;
while(low <=high)
{
int mid = (low +high)/2 ;
if(checkIf(mid , a , n, c ))
{
ans = mid ;
low = mid +1 ;
}
else high = mid-1 ;
}
cout<<ans<<nl;
}
int main() {
//testcases
int t ;
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin>>t ;
while(t--)
{
anything();
}
return 0;
}
