— Do you know what Tom does all day?

— I know he spends at least as much time watching TV as he _____ his  lessons. [ ] 

A. is doing

B. does

C. spends in

D. does doing

进行扩声系统的扬声器布置时，应考虑扬声器发出的声音要比自然声源延迟（）。

A.1～5ms

B.5～10ms

C.10～20ms

D.5～30ms