单项选择题 A green buoy has a yellow triangle on it. This is a(n)（）.

单项选择题 A frame with two，or sometimes four，arms through which are threaded the guidelines and which is used to keep the drill stem and bit in line with the center opening in the temporary guide base is the（）.

单项选择题 A flooded leg on a liftboat would adversely affect the vessel’s stability underway by（）.