（）is formed when the droplets or ice crystals in clouds have become sufficiently large to fall to the ground.

单项选择题（）reters to be amount of water vapour in the air.

A.Humidity
B.Temperature
C.Precipitation
D.Wind

单项选择题（）is used for receiving maritime safety information which is automatically printed by the receiver.

A.NAVETEX receiver
B.Search and Rescue Transponder
C.Emergency Position Indicating Radio Beacon
D.Digital selective calling

单项选择题（）is used for providing homing signals from survival craft for detection by 9Ghzradar.

A.NAVETEX receiver
B.Search and Rescue Transponder
C.Emergency Position Indicating Radio Beacon
D.Digital selective calling

单项选择题（）is used for calling and replying,and for transmitting acknowleding and relaying distress alerts.

A.NAVETEX receiver
B.Search and Rescue Transponder
C.Emergency Position Indicating Radio Beacom
D.Digital selective calling

单项选择题（）provides a paper trace of course steered by the vessel against time.

A.The echo sounder
B.The radar
C.The course recorder
D.The direction finder