—Are these computers?
—Yes , ____. [ ]
A. these're
B. those are
C. they are
D. they're
答案:C
电弧电压反馈调节系统的原理是什么?
配电线路敷设在有可燃物的闷顶内时,应采取穿()等防火保护措施。
A.金属管
B.PVC管
C.阻燃管
D.难燃材料管