一个人用动滑轮把重400N的货物提高4m,则有用功是______ J;若已知人所做的总功为2000J,则该装置的机械效率是________.
1600;80%
The growing interest in ____ history is clearly evident in ____ number of people visiting museums and country houses.
A.不填;不填
B.不填;the
C.the; the
D.the; 不填
体育教学中学练法包括()。