单项选择题
Before a tank is to be crude oil washed，the oxygen content in the tank must be measured at a position（）.
A.immediately above the level of the oil B.at the top of the tank C.in the vent riser D.one meter from the deck
单项选择题 Backfire flame arrestors are installed on（）.
单项选择题 Automatic fire dampers in ventilation systems are operated by use of（）.
单项选择题 Automatic fire dampers in ventilation systems are operated by a（）.
