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单选题
539、If a supply shutoff valve was closed for less than five minutes no maintenance is required and shutoff periods are not accumulative.
A
When a supply shutoff valve was closed for five/minutes, maintenance action is hardly required and shutoff duration are0 not included.
B
If a supply shutoff valve was closed within five minutes, no maintenance performance is needed land shutoff periods are not accumulated.
C
Maintenance action is not necessary and shutoff periods are not calculated unless the shutoff valve was closed for less than five minutes
答案解析
正确答案：B
解析：
解析:less than 少于,小于。within 在……之内。unless 除非,除……之外。accumulative 累积。calculated 计算。
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