Question 123
Suppose $f(x) = x^3 + \log_2(x + \sqrt{x^2+1})$. For any $a, b \in \mathbb{R}$, to satisfy $f(a) + f(b) \ge 0$, the condition $a + b \ge 0$ is ( ).
(A) necessary and sufficient
(B) not necessary but sufficient
(C) necessary but not sufficient
(D) neither necessary nor sufficient
(A) necessary and sufficient
(B) not necessary but sufficient
(C) necessary but not sufficient
(D) neither necessary nor sufficient