单项选择题
Fusible-link fire dampers are operated by（）.
A.8the heat of a fire melting the link B.a break-glass and pull-cable system C.electrical controls on the bridge D.a mechanical arm outside the vent duct
单项选择题 Foam-type portable fire extinguishers are most useful in combating fires involving（）.
单项选择题 Foam is a very effective smothering agent and（）.
单项选择题 Foam extinguishes a fire mainly by（）.
