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
  
  

Task 2

21. In most cases of cloud-to-ground lightning, the ground's surface__________

22. One form of lightning that is ball lightning. __________

23. Cloud lightning looks like a ribbon when its lighting channel__________

24. Although not fully understanding processes of lightning, man__________

25. Lightening can__________

A. occurs most infrequently

B. is shifted sideways by strong winds

C. strike twice or more in the same spot

D. is equipped with a good knowledge of various forms of lightning

E. is estimated at 20 million a year

F. is positively charged

The passage is mainly concerned with the

• A、types and function of sleep
  
  

• B、psychological role of dreams
  
  

• C、 history of reptilian sleep patterns
  
  

• D、influence of evolution on young animals