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

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单项选择题

The ARPA may swap targets when automatically tracking if two targets（）

A.are tracked on reciprocal bearings
B.are tracked at the same range
C.are tracked on the same bearing
D.pass closely to each other

单项选择题

Vapor given off by a flammable liquid will burn when ignited provided（）

A.the vapor and air mixture is too rich
B.it is mixed with certain proportions of air
C.the vapor is too little compared to the air
D.it is of heavy toxicity

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Fixed piping leads from the（）to various areas of the compartment to be flooded.

A.manifold
B.remote pull box
C.discharge nozzle
D.machinery space