使用分离器必须遵守（）原则进行操作。
A、先开后关
B、先关后开
C、先关后开不憋压
D、先开后关不憋压
参考答案：D
悲哀在认知方面的反应表现为( )
A．痛苦、愤怒、焦虑
B．头疼、失眠
C．心不在焉、健忘
D．睹物思情、行为怪异
Why did the United States first launch satellites into space a. To improve navigation for warplanes and warships. b. To guide smart bombs that home in on moving targets. c. To collect meteorological data. d. To sense remote volcanic eruptions.
