单项选择题
A.once each crankshaft revolution
B.twice each crankshaft revolution
C.every other crankshaft revolution
D.every fourth stroke
单项选择题 Centrifugal pumps have what advantage(s) over reciprocating pumps?（）.
单项选择题 Cargo pump relief valves are piped to the（）.
单项选择题 As the propeller turns，voids are formed on the trailing and leading edges of the propeller blades causing a loss of propulsive efficiency，pitting of the blades，and vibration. These voids are known as（）.