According to the passage, positive ions can be produced by ______.

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

A．Positive and Negative Ions
B．Methods of the Prediction of Earthquakes
C．Ionizer: Best Way to Improve Your Health
D．Fatigue and Headache: Plagues of Positive Ions