单项选择题 A (n) （） is a chemical or electro-chemical attack on the metal surface which may be further increased if the sea-water is polluted.

单项选择题 A propeller of 5~65m in diameter, having a mean pitch of 4~88m, turning at 72 rpm, at a speed of 11 knots (339~79m/min), will give （） of apparent slip.

单项选择题 In moving forward, a ship imparts forward motion to the water at the stems, this is known as （）.