To which subset of real numbers does - 18 not belong?

The subsets of the real numbers are the irrational numbers, rational numbers, integers, whole numbers, and the natural numbers. First, figure out where does - 18 belong? Well, - 18 is a rational number because it can be expressed as a fraction of two integers. It's also an integer.

So, it doesn't belong in the sets of irrational numbers, whole numbers, and the natural numbers.