设$0 < a_{1} < \pi$ ,$a_{n+1}= \sin a_{n}(n=1,2, \cdots )$
.(1)证明: $\lim _ {n \rightarrow \infty }a_{n} $存在，并求此极限；
(2)求 $\lim _ {n \rightarrow \infty } \left ( \dfrac {1}{a_{n+1}^{2}}- \dfrac {1}{a_{n}^{2}} \right )$.
$\text{A.}$
$\text{B.}$
$\text{C.}$
$\text{D.}$