2.1.1 * Let $x \in\left(-\frac{1}{2}, 0\right)$, then the size relationship of $a_{1}=\cos (\sin x \pi), a_{2}=\sin (\cos x \pi), a_{3}=$ $\cos (x+1) \pi$ is ().
(A) $a_{3}<a_{2}<a_{1}$
(B) $a_{1}<a_{3}<a_{2}$
(C) $a_{3}<a_{1}<a_{2}$
(D) $a_{2}<a_{3}<a_{1}$