Question

Which statement is true for the set of natural numbers?

A. The set is closed under addition and closed under subtraction
B. The set is closed under addition and not closed under subtraction
C. The set is not closed under addition and closed under subtraction
D. The set is not closed under addition and not closed under subtraction

Answer 1

It's closed under addition. You can take any two natural numbers you want, add them up and you'll get some other natural number (eg: 3 and 5 ---> 3+5 = 8)

it's not closed under subtraction. Sure 5-3 = 2 is a natural number but 3-5 = -2 is not a natural number

so that leads us to the answer being choice B