Besidesheat,fuel,andoxygen,whichofthefollowingisnecessaryforafiretoexist?（）
A. Chainreaction
B. Thermalaction
Chemicalocclusion
D. Pyrotechnicalreaction
Whichofthelistedclassesoffireisconsideredasoneinvolvingelectricalequipment?（）
A. Class"A"
B. Class"B"
Class"C"
D. Class"D"
在相同起升高度情况下，双箱形结构主梁的桥吊总高度比单箱梁要小（）m左右。
A. 1
B. 2
C. 3
D. 4
Avesselisheadingmagneticeastanditsmagneticcompassindicatesaheadingof086°.Whichactionshouldbetakentoremovethiserrorduringcompassadjustment?（）
A. Iftheblueendsofthemagnetsareaft，andthefore-and-afttrayisatthetop，youshouldaddsomemagnets.
B. Iftheblueendsofthemagnetsareaftyoushouldlowerthefore-and-afttray
C. Iftheblueendsofthemagnetsareaft，andthefore-and-afttrayisatthetop，youshouldreversethemagnets
D. Iftheblueendsofthemagnetsareforward，andthefore-and-afttrayisatthebottom，youshouldaddsomemagnets
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