单项选择题
A vessel behaves as if all of its weight is acting downward through the center of gravity，and all its support is acting upward through the（）.
A.keel B.center of buoyancy C.tipping center D.amidships section
单项选择题 A spark arrestor（）.
单项选择题 A relief valve for a cargo pump is generally installed（）.
单项选择题 A gas-free certificate would usually be issued by a(n)（）.
