单项选择题
为做好信号机维护工作,每个信号灯每年至少关闭一次,持续(),其间对应的SOM6上所有灯都应关闭。
A.60s
B.120s
C.180s
D.300s
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单项选择题
A.60s
B.120s
C.180s
D.300s
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