单项选择题
（）is formed when the droplets or ice crystals in clouds have become sufficiently large to fall to the ground.
A.Humidity B.Temperature C.Precipitation D.Wind
单项选择题 （）reters to be amount of water vapour in the air.
单项选择题 （）is used for receiving maritime safety information which is automatically printed by the receiver.
单项选择题 （）is used for providing homing signals from survival craft for detection by 9Ghzradar.
