问题
单项选择题
On offshore drilling units，emergency lighting and each emergency power system must be tested at least once a（）.
A.day
B.week
C.month
D.year
答案
参考答案：B
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On offshore drilling units，emergency lighting and each emergency power system must be tested at least once a（）.
A.day
B.week
C.month
D.year
参考答案：B