单项选择题
A.displaying code letter Y (Yankee)
B.sounding five short blasts on his whistle
C.displaying code numeral 5
D.sounding one prolonged blast on his whistle
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（）.
单项选择题 A vessel shall slacken her speed，stop，or reverse her engines，if necessary，to（）.