Which is the best definition of closure?
Number sets are closed under an operation when you perform the operation and the result is a number that is not a real number.
Number sets are closed under an operation when you perform the operation and the result is a member of the opposite number set.
Number sets are closed under an operation when you perform the operation and the result is a member of a related number set.
Number sets are closed under an operation when you perform the operation and the result is another member of the same number set