单项选择题
Questions 20 to 22 are based on the conversation you have just heard.
A. That a snow cover can cause ice to melt. B. How heat can prevent ice from melting. C. How water flows into the ocean. D. Why volcanoes have a slippery surface.
A. Research assistants. B. Magazine publishers. C. Careless authors. D. Inexperienced students.
A. Its large number of bicycle clubs. B. Its geographic variety. C. Its network of superhighways. D. Its mild climate.