A:were reading
B:had read
C:have read
D:read
据统计,自上古至隋朝见于记载的88件水利工程中,华北占72件,华南仅占16件。而唐代见于记载的212件水利工程中,华北占101件,华南占11l件。这说明
— Shall Brown come and play computer games?
— No, ________ he has finished his homework.
用图甲、乙两种方式匀速提升重为100N的物体,已知滑轮重20N、绳重和摩擦力不计。则( )