We went to the West Lake yesterday, ____, not surprisingly, was crowded with visitors from all

over the world. [ ]

A. where

B. which

C. that

D. when

物体从距凸透镜12cm移到距凸透镜18cm的过程中，调整光屏的位置，总能在光屏上得到倒立放大的像，由此可知，此凸透镜的焦距可能是（    ）

A．6cm

B．10 cm

C．11 cm

D．20 cm