单项选择题
0pen ullage holes in tanks which are not gas-free must be protected by（）.
A.PV valves B.warning signs C.flame screens D.stop-check valves
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（）.
单项选择题 Paints and solvents on a vessel should be（）.
