单项选择题 A vessel which is being assisted by an icebreaker should indicate that she is ready to cast off the towline by（）.

单项选择题 A vessel the passage of which is not to be impeded（）to comply with this Rule when the two vessels are approaching one another so as to involve risk of collision.

单项选择题 A vessel showing a rigid replica of the International Code flag A is engaged in（）.