单项选择题
Which display indicates a vessel conducting mineclearance operations（）.
A.Three balls in a vertical line B.Two balls in a vertical line C.One ball near the foremast and one ball at each yardarm D.One diamond near the foremast and one ball at each yardarm
单项选择题 When you doubt the existence of risk of collision,（）.
单项选择题 When visibility is restricted（），we can say it is restricted visibility.
单项选择题 When underway in a channel，you should keep to the（）.
