单项选择题
After being launched from a vessel，totally enclosed survival craft which have been afloat over a long period require（）.
A.frequent opening of hatches to permit entry of fresh air B.regular checks of bilge levels C.use of ear plugs to dampen engine noise D.frequent flushing of the water spray system with fresh water
单项选择题 Admiralty EasyTide（）.
单项选择题 Aboard a survival craft，ether can be used to（）.
单项选择题 A vessel in distress should send by radio telephone the two tone alarm signal followed immediately by the（）.
