True or false? Negative numbers are closed under addition. Give a counterexample

Question

True or false? Prime numbers are closed under subtraction. Give a counterexample
True or false? Natural numbers are closed under division. Give a counterexample

Answer 1

I'll do one; you try the others.

True or false? Negative numbers are closed under addition.

True: for any positive a,b
-a + -b = -(a+b)

Answer 2

False