单项选择题
Which of the following is not a factor which can interfere with the pulse of sound and cause a variation the depth reading?（）
A.salinity B.temperature C.the hydrostatic pressure D.Vessel’s speed
单项选择题 Who can contact the Designed Person?（）
单项选择题 When shall the stand-on vessel change course and speed?（）
单项选择题 Carbon dioxide flooding system is used for extinguishing fires because it reduces the concentration of oxygen in the air to（）.
