① Let's go home! ____ A. It's 9 o'clock.
② Where is Amy? ____ B. Yes.I do.
③ What time is it now? ____ C. OK.
④ Do you like music class? ____ D. She is in the art room.
如图1所示,已知小灯泡L上标有“6V 3W”字样,R0=20Ω,滑动变阻器R的最大阻值为100Ω。求: