关于Rb，说法正确的是（）.

A.内生性Rb起源于视网膜外层，易造成视网膜脱离

B.外生性Rb起源于视网膜内层

C.Rb主要起源于神经元，同时伴有光感受器细胞分化

D.Rb基因属于致癌基因

E.眼底见黄白色隆起肿块，表面无血管分布

With a good command of reading skills, most students can manage to read（）as they could the year before.

A.as twice fast

B.as fast as twice

C.as twice as fast

D.twice as fast