The author states that during dream sleep, the animal sleeps

• A、without moving
  
  

• B、shallowly
  
  

• C、intermittently
  
  

• D、on its side
  
  

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

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