Answer:
a, b and a - b all come from the same set
Step-by-step explanation:
"Closure" for an operation means that the two numbers and the answer after the operation is done all come from the same set of numbers.
Example: The set of positive integers (counting numbers) is closed under addition -- (positive integer) + (positive integer) = (positive integer)
But the set of positive integers is not closed under subtraction -- 8 - 10 = (not a positive integer)
