A:13
B:12
C:34
D:23
如图所示的滑轮组,每个滑轮重为20N,绳重及摩擦不计.用它匀速提升重为100N的物体,所用的拉力F是( )
Mr. Black has a daughter. Tod ay is her ____ (twelve) birthday.
关于内环境稳态调节的现代观点是( )