单项选择题
On a hydrocarbon flammability chart the line which extends from 0% to 21.8% oxygen，lying tangent to the flammability range，is called the（）.
A.minimum oxygen content line B.critical displacement line C.critical dilution line D.upper threshold limit
单项选择题 If，during helicopter refueling operations，fuel is spilled on clothing，the person should first（）.
单项选择题 Each part of the fire-main system located on an exposed deck must be（）.
单项选择题 Each hose in the fuel transfer system for helicopter refueling must meet the standards of the（）.
