The author suggests that, because there are no hiding places in the oceans, whales and dolphins

• A、 sleep only when they are very young
  
  

• B、 cannot afford to sleep very much
  
  

• C、usually dream while awake
  
  

• D、take turns to sleep
  
  

Task 1

16. Paragraph     ①     

17. Paragraph     ②     

18. Paragraph     ③     

19. Paragraph     ④     

20. Paragraph     ⑤     

A. Lightening Causes Puzzles to People

B. Types of Lightning

C. Causes of Lightning

D. Differences Between Thunder and Thunderstorm

E. Frequencies of Thunderstorms Occurring in the World and the U. S.

F. Shock Waves as Thunder

It can be inferred from the passage that animals that are prey rarely experience dream sleep because they

• A、have evolved further than predators
  
  

• B、are particularly intelligent animals
  
  

• C、need less sleep than predators
  
  

• D、would be caught if deeply asleep