Questions 16 to 19 are based on the talk in class about United States history.

　　16. What is the main topic of the talk?

　　17. According to the professor, what was the major change in agriculture during the 19th century?

　　18. According to the professor, what was one result of the increased use of machinery on farms in the United States?

　　19. According to the professor, why was world market important for the United States agriculture?

　　Passage 6

　　The energy which the sun radiates goes in every direction. However, only a minute part of it falls on the earth. Even so, it represents the power of about five million horsepower per square mile per day. The sun gives us as much energy every minute as mankind uses in a year. At present, we use this energy indirectly, and it is our only final source of power. Coal represents the chemical action of the sun on green plants thousands of years ago. Water power results from the sun's creation vapor and the resulting rain. Even windmills operate because of air currents set in motion by the uneven heating effects of the sun in different places, Already, a scientist has worked out an engine, surprisingly efficient. This engine uses a series of mirrors which concentrate the sun's energy on a tube of water to create seam.

　　Questions 20 to 22 are based on the passage you've just heard.

　　20. What does this passage tell us about?

　　21. How much energy does the sun give to the earth in a minute?

　　22. How do people use solar energy at present?

　　題解：

　　Passage 5

　　這是老師在課堂授課的講稿。文章介紹了19世紀美國農民如何從自給自足到依賴他人的演變過程。分析了造成這一變化的原因。

　　聽時要注意按照時間順序理清農民的特點及原因。

　　16.D 這是一道主旨題。把握時間的順序，如at the beginning of the 19th century, but by the mid century, by late in the century便可知。文章的最后一句話也說明了這層意思。

　　17.A 這是一道細節題。從"revolutionary advances ... has vastly increased production of specialized crops .... "可知。

　　18.B 這是一道細節題。從"Now farmers were no longer dependent ..., their lives were increasing controlled by banks, which had powder to grant or deny loans for new machinery."可知。