Let $f ( x ) = \sin x + \alpha x , x \in \mathbb { R }$, where $\alpha$ is a fixed real number. The function $f$ is one-to-one if and only if\\
(A) $\alpha > 1$ or $\alpha < - 1$.\\
(B) $\alpha \geq 1$ or $\alpha \leq - 1$.\\
(C) $\alpha \geq 1$ or $\alpha < - 1$.\\
(D) $\alpha > 1$ or $\alpha \leq - 1$.