玻璃板液面计的作用？

A. change course twenty degrees to the left
B. put the rudder left twenty degrees
C. put the rudder hard left for the first twenty degrees of swing
D. put the rudder left twenty degrees and then ease back as the vessel starts swinging

A. 遮蔽灯光不使其外露
B. 采用不耀眼的弱光
C. 采用不耀眼的红光
D. ABC都对