8 Given the function $f(x)=x^{3}-\log _{2}\left(\sqrt{x^{2}+1}-x\right)$, then for any real numbers $a, b$ $(a+b \neq 0), \frac{f(a)+f(b)}{a^{3}+b^{3}}$ is ( ).
(A) always greater than zero
(B) always equal to zero
(C) always less than zero
(D) the sign is uncertain