#include <bits/stdc++.h>
#define ll unsigned long long int
using namespace std;
ll tc;
 ll n,c, x;
        vector<ll> a;
int main()
{
    ios_base::sync_with_stdio(0);cin.tie(NULL);cout.tie(NULL);
    
    cin>>tc;
    while(tc--)
    {
       
        cin>>n>>c;
        
        for(int i=0;i<n;i++)
        {
          cin>>x;
          a.push_back(x);
        }  
        sort(a.begin(),a.end());
        ll i=1,j=1000000001,ans=0,l=0;
        while(i<=j)
        {
            ll mid=((j+i)/2);
            ll fi=a[0],temp=1;
            for(int k=1;k<n;k++)
            {
                if(a[k]-fi>=mid)
                   { temp++;
                    fi=a[k];
                    }
            }
            if(temp<c)
                j=mid-1;
            else
            {
                ans=mid;
                i=mid+1;
            }
        }
        cout<<ans<<endl;
        a.clear();
    }
    return 0;
}