[单项选择]Questions 62 to 66 are based on the following passage.
If a mother pushes her small son in a swing (秋千), giving only a light force each time he returns, eventually he will be swingirg.quite high. The child can do this for himself by using his legs to increase the motion, but both the mother’s push and the child’s leg movements must occur at the proper moment, or the extent of the swing will not increase. In physics, increasing the swiag is increasing the amplitude (振幅); the length of the rope on the swing determines its natural oscillation (摆动) period. This ability of an object to move periodically or to vibrate when stimulated by a force operating in its natural period is called resonance.
Resonance is observed many times without consciously thinking about it; for example, one may find an annoying vibration or shimmy in an automobile, caused by a loose engine mount vibrating with increasing amplitude because of an out of-round tire. The bulge (凸出部分) on t
A. the speed of a motor in revolutions
B. the frequency at which a motor vibrates
C. the amplitude of an engine that oscillates
D. the changes in a dial within a ear engine