问题
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
答案
参考答案：C
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（）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
参考答案：C