（）sends a radio signal from the bottom of the ship to the sea-bed,from which it is reflected.

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

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

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