单项选择题
（）is an equipment used to detect the presence of ships ,buoys,coast and many other targels and to obtain range and bearing of these targets.
A.The marine sextant B.The chronometer C.The echo sounder D.The radar
单项选择题 （）sends a radio signal from the bottom of the ship to the sea-bed,from which it is reflected.
单项选择题 （）is the navigational timekeeper of the vessel.
单项选择题 （）is used for measuring horizontal and vertical angles.
