Answer:
It is may be earthquakes.
Prevalent synonym: widespread
Requisite synonym: necessary
Propagate synonym: spread
Hope this helps :)
The correct answer would be option A. "Paragraph one explains the cause of investigation and Paragraph two shows the results". This is because in the first paragraph the author is letting you know about an event that no one knew the reason for its appearance (so it is a cause for investigation) and in the second paragraph the author explains the results of the research conducted to explain said event. Option B is not correct because the first paragraph would be the problem and the second paragraph would be the solution. Option C is not correct either because the second paragraph is more than facts, it is answers. And finally, Option D is not correct because there is a clear connection between both paragraphs (since they are about the same subject).