在亚声速流动时,管道截面缩小,气流速度()
A. 增加
B. 减小
C. 不变
驱动液压泵的电机功率应比液压泵的输出功率大,是因为()。
A. 泄漏损失
B. 摩擦损失
C. 溢流损失
D. 前两种
Chinese rules of etiquette regarding costumes and adornments started taking shape in the______ dynasty, regulating from nobles down to commoners.
A. Xia
B. Shang
C. Zhou
D. Han
_____ had been used in the front of costumes for several thousand years before the Ming dynasty.
A. buttons
Band knots
C. belts
D. scarf