Don't do any other things while you are doing your homework. So please _____ your MP4.[ ]

A. turn up  

B. turn down    

C. turn off    

D. turn on

如图所示，硬质杠杆AOB上挂一重物G，O为支点，请你在杠杆AOB上作出使重物在图示位置静止时的最小力F及其力臂l。