Enthuware

I checked point 1. Namely, I created the Furnace class in the same com.cool package. My class looks like this:

package com.cool;
public class Furnace
{
public void cool(Cooler c) { // 2
c.doCool();
}
}

The compiler didn't throw any errors. Thus 3 correct answers: 1,3,6. But only 3.6 are selected as correct

True, if you put the Furnace class in com.cool package, it will compile. But it is not a right answer because the problem statement says that Cooler is part of a library that has been given to you. Furnace is your class and it uses this library. It is not a part of that library. It implies that it is not in com.cool package.