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
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用来表征物质导磁性能的物理量称为()。
A. 磁导率
B. 磁通量
C. 磁感应强度
D. 磁力线