#include
#include
using namespace std;
bool canplacecow(int stall[],int n, int c, int min_space) {
int last_cow = stall[0];
int cnt = 1;
for(int i=1;i<n;i++) {
if(stall[i]-last_cow>=min_space) {
last_cow = stall[i];
cnt++;
if(cnt==c) {
return true;
}
}
}
return false;
}
int main() {
// your code goes here
int t,n,c,ans=0;
int stall[100000];
cin >> t;
while(t–) {
cin >> n >> c;
for(int i=0;i<n;i++) {
cin >> stall[i];
}
sort(stall,stall+n);
int s=0;
int e=stall[n-1] -stall[0];
while(s<=e) {
int mid=(s+e)/2;
bool cowrakhpaye = canplacecow(stall, n,c,mid);
if(cowrakhpaye) {
ans=mid;
s=mid+1;
}
else {
e=mid-1;
}
}
}
cout << ans << endl;
return 0;
}