What caused the blackout in July 1977?
A.Excessive heat probably made people switch on too many electrical appliances.
B.Because of unemployment, some machines were not in proper working order.
C.During a storm, lightning damaged supply cables.
D.The passage does not mention the cause.
第2题
如在()的平台上工作,为了防止工具和器材掉落,应采取有效隔离措施,如铺设木板等。
A 格栅式
B 全封闭
C 检修专用
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