Yes it has closure, identity, inverse, and an associative property.
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Subtraction.
To be closed under an operation, when that operation is applied to two member of a set then the result must also be a member of the set. Thus the sets ℂ (Complex numbers), ℝ (Real Numbers), ℚ (Rational Numbers) and ℤ (integers) are closed under subtraction. ℤ+ (the positive integers), ℤ- (the negative integers) and ℕ (the natural numbers) are not closed under subtraction as subtraction can lead to a result which is not a member of the set.
They are closed under all except that division by zero is not defined.
Because both of those numbers are rational. The sum of any two rational numbers is rational.
No.