A、If we wake fast sleeping people, they will feel annoyed. B、If we wake people during the REM, they will feel tired. C、Only drea

[19]Man has always been fascinated by dreams. He has always tried to find explanations for his dreams. Perhaps dreams tell us about the future or the past; perhaps they tell us about our deepest fears and hopes. I’ll give you one or two facts about dreams. First of all, everyone dreams. You often hear people say, "I never dream" when they mean "I can never remember my dreams." When we dream, our eyes move rapidly in our sleep as if we were watching a moving picture, following it with our eyes. [20]This movement is called REM, which is Rapid Eye Movement REM sleep is the sleep that matters.
   [21]Experiments have proved that if we wake people up throughout the night during REM, they will feel exhausted the next day. But they won’t feel tired at all if we wake them up at times when they’re not dreaming. So it is clear that it is dreaming that really refreshes us, not just the sleep.
Questions 19 to 21 are based on the passage you have just heard.
19. Why has man been so fascinated by dreams?
20. What does REM mean?
21. What have the experiments proved about dreams?