單項(xiàng)選擇題For a fire to begin there must be present a combustible material, oxygen or air to ()combustion, and a source of heat at a temperature high enough to start combustion.

A.begin
B.support
C.last
D.extinguish


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5.單項(xiàng)選擇題() the oxygen content in the exhaust gas, () the combustion is.

A.The lower;the worse
B.The higher;the better
C.The lower;the better
D.The highest;the worst