I like to have my umbrella _____________ a rainy day. [ ]
A. at
B. on
C. in
答案:B
活塞泵出液量少的原因有哪些?
列车制动进行动能转换的过程中,每单位时候内转移的列车动能称为()
A.制动率
B.制动功率
C.制动倍率