Question

Let S be the set of all non-negative multiples of 4 (0,4,8,12 and so on). S is closed with respect to which ordinary operation?

Answer 1

multiplication, since all products will be divisible by 4.

addition as well, since you can write these sums as the products of 4.

Not subtraction, because you might obtain a negative answer

Lastly, not division, because you might obtain 4/4=1, which is not part of the set.