题目内容

在超声速流动时,管道截面扩大,气流速度()。

A. 增加
B. 减小
C. 不变

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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

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