He studied very hard all the time because he didn’t want to __________in his studies.

A．fall behind

B．be left behind

C．left behind

D．Both A and B

当要求两轴轴心较近，传动精确时可采用（）。

A.平皮带传动

B.齿轮传动

C.三角皮带传动

D.液压传动