On a hydrocarbon flammability chart the line which extends from 0% to 21.8% oxygen，lying tangent to the flammability range，is called the（）.

If，during helicopter refueling operations，fuel is spilled on clothing，the person should first（）.

A.see the medic immediately
B.spray himself with foam or CO₂
C.complete his task and then see the medic
D.remove the clothing and wash

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Each part of the fire-main system located on an exposed deck must be（）.

A.protected against freezing
B.locked to prevent theft
C.numbered sequentially
D.pressurized at all times

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