Which display indicates a vessel conducting mineclearance operations（）.

单项选择题When you doubt the existence of risk of collision,（）.

A.such risk shall be deemed to exist
B.such risk shall not be deemed to exist
C.you needn't take any action to avoid collision with any vessel
D.you should accelerate your speed ahead

单项选择题When visibility is restricted（），we can say it is restricted visibility.

A.by long distance
B.by the darkness at night
C.by a vessel in front
D.by fog or falling snow

单项选择题When underway in a channel，you should keep to the（）.

A.Middle of the channel
B.Starboard side of the channel
C.Port side of the channel
D.Side of the channel that has the widest turns

单项选择题When underway in a channel，you should if safe and practicable（）.

A.stay near the middle of the channel
B.keep to the starboard side of any vessels you meet
C.exchange whistle signals with any other vessels in the channel
D.keep to the side of the channel which lies to your starboard

单项选择题When two vessels are in sight of one another and NOT in or near an area of restricted visibility，any of the following signals may be given EXCEPT（）.

A.a light signal of at least five short and rapid flashes
B.one prolonged，one short，one prolonged，and one short whistle blasts
C.four short whistle blasts
D.two short whistle blasts