单项选择题
Passage One Questions 27 to 29 are based on the passage you have just heard.
A. Always use a loudspeaker. B. Avoid large rooms. C. Never vary the volume. D. Not to shout.
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.