单项选择题
A.foul of anchor B.dropping anchor C.weighing anchor D.walking back anchor
（）sends a radio signal from the bottom of the ship to t...
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
The ARPA may swap targets when automatically tracking i...
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
Vapor given off by a flammable liquid will burn when ig...
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
