用显微镜观察细胞的结构时，所要观察的材料必须是透明的，其原因是[ ]

A．能让光线透过

B．便于放置在显微镜上

C．不易污染物镜

D．易于染色

4．Tom told me that after a few years in China not only____his Chinese, and that he had also obtained more interest.

A．does he improve

B．had he improved

C．he had improved

D．he will improve