单项选择题
An instrument that indicates wind direction is known as a(n)（）.
A.weather vane，wind vane or wind sock B.hydrometer C.hygrometer D.sling psychrometer
单项选择题 All of the following steps are taken in starting a centrifugal pump，EXCEPT to（）.
单项选择题 After an engine is started you should（）.
单项选择题 A wobbling tail shaft is an indication of（）.
