单项选择题
0ne of the principal dangers inherent in liquified petroleum gas is（）.
A.as it warms up it becomes heavier than air B.the way it reacts with sea water C.the strong odor it produces D.its low temperature causes frostbite or freezing
单项选择题 0pen ullage holes in tanks which are not gas-free must be protected by（）.
单项选择题 0ur starboard derrick boom of hatch No.4 is baddly cracked（）that it has become entirely useless.
单项选择题 0vercarring is best prevented by（）.
