The laughter at his joke is never forced, but always ______.

A．fluctuate
B．fabricate
C．lubricate
D．manipulate

A．ionizers don' t have a positive effect on people' s health
B．a feeling of well-being is accounted for only by the presence of' waterfalls or fountains
C．usual animal behavior will predict the coming of a major earthquake
D．whether seismic readings are as effective as watching animal behavior in predicting an earth- quake is not certain