单项选择题
Combustible gas indicators operate by drawing an air sample into the instrument（）.
A.over an electrically heated platinum filament B.where it is mixed with nitrogen C.where it is ignited by a sparking device D.where its specific gravity is measured
单项选择题 Butadiene，inhibited，is labeled as a（）.
单项选择题 Before starting a gasoline engine on a motorboat，you should make sure for safety that（）.
单项选择题 Before entering a tank，always check the vapor content（）a combustible gas indicator.
