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 （）is the navigational timekeeper of the vessel.
单项选择题 （）is used for measuring horizontal and vertical angles.
单项选择题 （）means rudder to be held in the force and aft angles.
