8. The function $f(x)(x \in \mathrm{R})$ is not always greater than 0, and satisfies $f(a+b)=f(a)+$ $f(b)$, then $f(x)$ is
(A) even function
(B) odd function
(C) both an odd function and an even function
(D) neither an odd function nor an even function
B

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