____ you take a photo, you should always check the position of the sun. [      ]

A. Before            

B. After                  

C. Because            

D. Though

急性闭角型青光眼最主要的发病机制为（）。

A.眼轴短

B.小角膜

C.小梁网功能不全

D.房角窄

E.瞳孔阻滞