8. Given the function
$$
f(x)=x^{3}-\log _{2}\left(\sqrt{x^{2}+1}-x\right) \text {. }
$$

For any real numbers $a, b (a+b \neq 0)$, the value of $\frac{f(a)+f(b)}{a^{3}+b^{3}}$ is ( ).
(A) always greater than zero
(B) always equal to zero
(C) always less than zero
(D) the sign is uncertain