The set of odd and even positive integers closed under multiplication

1.	The set of odd and even positive integers closed under multiplication is ________
A.	a free semigroup of (M, ×)
B.	a subsemigroup of (M, ×)
C.	a semigroup of (M, ×)
D.	a subgroup of (M, ×)
Answer» C. a semigroup of (M, ×)

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