TOEFL Listening: TPO-TOEFL听力TPO - B240UMP97YJGFAGCR$

Why does the professor describe moving a sheet of heavy paper over a candle? A. To clarify that plumes do not produce great amounts of heat B. To describe an experiment he would like the students to conduct C. To illustrate one hypothesis for the way some volcanic anomalies are formed D. To emphasize the thinness of some of Earth's tectonic plates