I thought Tom was dropping his head low______ the text, _____, in fact he was sleeping. [ ]

A. to read,  when                  

B. to be reading,  since  

C. read,  where                    

D. reading,   while

屈光度为2.00D的薄透镜，物在透镜前2m处，像在透镜后（）处。

A、0.67m

B、3.00m

C、1.00m

D、0.47m