体操运动员优美的动作都是通过神经系统的支配，使   收缩和舒展，牵引所附着的    ，绕着    活动而形成的。

I wanted to know ______ 1,600 words as soon as possible. 　　[ ]

A. how can they learn　　　　

B. how could they learn 　　

C. how they can learn 　　　　

D. how they could learn