单项选择题
A chemical additive to LPG gives it a characteristic（）.
A.odor
B.color
C.pressure
D.density
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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.mimimum oxygen content line
B.critical displacement line
C.critical dilution line
D.upper threshold limit
