#include <bits/stdc++.h>
 
 
#define fl(i,n)    for(int i = 0; i < n; i++)
 
#define ll   long long
#define nl   endl
#define pb   push_back
#define mp   make_pair
#define fi   first
#define se   second
#define PII  pair<int,int>
 
#define EPs  1e-9
#define INF  1e16
 
#define setBit(i,n)     (i|1<<n)
#define clearBit(i,n)   (i&(~(1<<n)))
#define toggleBit(i,n)  (i^1<<n)
#define isOn(i,n)       ((i&1<<n) != 0)
 
 
using namespace std;
 
 
 
ll n, c;
ll a[1000000];
 
 
bool check(ll x){
    ll cows = 0, last = a[0];
    cows++;
    for(int i = 1; i < n; i++){
        if(a[i]-last >= x){
            cows++;
            if(cows == c) return true;
            last = a[i];
        }
    }
    return false;
}
 
 
ll binsearch(){
    ll low = 0, high = INF, mid;
    while(low+1 < high){
        mid = (low+high)>>1;
        if(check(mid)){
            low = mid;
        }
        else
            high = mid;
    }
    return low;
}
 
 
 
int main()
{
    int t;
    cin >> t;
    while(t--){
        cin >> n >> c;
        for(int i = 0; i < n; i++) cin >> a[i];
        sort(a,a+n);
        cout << binsearch() << nl;
    }
 
 
    return 0;
}
 

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