- do I need to feed the pet dog?
- Twice a day.
A:How long
B:How much
C:How soon
D:How often
Claire knew that Gladys thought she was having an .
如图所示,一束光经过一个透镜后会聚于主光轴上的点A,取掉透镜后,光会聚于B点,则该透镜起到____作用(填“会聚”或“发散”),是____透镜。