单项选择题
A.Flows of fluids can be restricted by both cocks and valves
B.There are seats in the internal passages of valves
C.A disc of valve only moves up and down
D.We can infer that there is a bore in cock ping
单项选择题 It should be apparent that little or nothing is gained by opening a drain at the same time as or after admitting steamThe above sentence probably means （）.
单项选择题 It is usual to identify the fluids in pipes by bands of paint at intervals, and the engineer should familiarize himself with the code in use.According to the above sentence, which of the following statements is true?（）
单项选择题 Disastrous explosions have been caused by accumulations of oil or oil vapour in air lines not drainedAccording to the above sentence, which of the following statements is true?（）