The contrast drawn between the Haicheng earthquake in Paragraphs 2 and 3 and the Tangshan earthquake in Paragraph 3 contributes to a central idea that earthquakes are A all preceded by reliable signs, and therefore scientists have the ability to predict earthquakes. B controlled by observable factors that just average citizens can report to the authorities. C not always predictable, so new methods and more studies need to be conducted. D not often studied because there is no possible solution to the problem of predicting earthquakes.​