#include<bits/stdc++.h>
using namespace std;
long long int arr[2000000],n,cow;

bool chk(long long int dis)
{
   // cout<<"dis "<<dis<<endl;
    long long int i,k=0,c=0,sum=1;
    for(i=0;i<n;i++)
    {
        if(arr[i]-arr[k]>=dis)
        {
            sum++;
            k=i;
            if(sum==cow)
                return true;
        }
    }
    return false;
}

int main()
{
    long long int t,i,j,ans,lo,hi,mid;
    cin>>t;
    while(t--)
    {
        cin>>n>>cow;
        for(i=0;i<n;i++)
            cin>>arr[i];
        sort(arr,arr+n);
        lo=0,hi=arr[n-1],ans=0;
        while(lo<=hi)
        {
            mid=(lo+hi)/2;
       //     cout<<"1st mid "<<mid<<endl;
            if(chk(mid))
            {
                ans=mid;
                lo=mid+1;
            }
            else
            {
                hi=mid-1;
            }
         //   cout<<"hi "<<hi<<endl;
           // cout<<"lo "<<lo<<endl;
           // cout<<"mid "<<mid<<endl;
           // cout<<"ans "<<ans<<endl;
        }
        cout<<ans<<endl;
    }
    return 0;
}