单项选择题
（）sends a radio signal from the bottom of the ship to the sea-bed,from which it is reflected.
A.The marine sextant B.The chronometer C.The echo sounder D.The radar
单项选择题 The ARPA may swap targets when automatically tracking if two targets（）
单项选择题 Vapor given off by a flammable liquid will burn when ignited provided（）
单项选择题 Fixed piping leads from the（）to various areas of the compartment to be flooded.
