The author states that during dream sleep, the animal sleeps

• A、without moving
  
  

• B、shallowly
  
  

• C、intermittently
  
  

• D、on its side
  
  

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 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