15
（1）changing the status of the adjustive valve
(2) changing the speed of the driving motor
(3) changing the diameter of the impeller （irreversible) （1）
16 A C
17
18 It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities where the pressure is relatively low. When subjected to higher pressure, the voids implode and can generate an intense shockwave.This shockwave will do harm to the impeller and causes metal fatigue on the surface of the impeller.
19 the viscosity will decrease
20