If $a$ and $b$ are integers and $a-b$ is even. which of the following must always be even?

(A) $a b$

(B) $a^{2}+b^{2}+1$

(C) $a^{2}+b+1$

(D) $a b-b$

If $a$ and $b$ are integers and $a-b$ is even. which of the following must always be even?

(A) $a b$

(B) $a^{2}+b^{2}+1$

(C) $a^{2}+b+1$

(D) $a b-b$

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