翻转客房床垫的周期一般是多久（）。
A.一个月
B.一个季度
C.半年
D.一年
参考答案：B
All of the following are distress signals EXCEPT（）.
A.the continuous sounding of any fog signal apparatus
B.giving five or more short and rapid blasts of the whistle
C.firing a gun at intervals of about a minute
D.a barrel with burning oil in it，on deck
两合公司
