Which set is closed under subtraction?

Rational numbers are closed under subtraction: subtracting any two rational numbers always gives another rational number. For example, 3/4 − 1/2 = 1/4, which is rational. Whole numbers and natural numbers are not closed under subtraction because, for example, 2 − 5 = −3, which is in neither set. Irrational numbers are not closed either; for example, √2 − √2 = 0, which is rational.

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