注意原文中出现的表示时间节点的词 before, after, not…until…
The story of the westward movement of population in the United States is, in the main,the story of the expansion of American agriculture-of the development of new areas for theraising of livestock and the cultivation of wheat, corn, tobacco, and cotton. After 1815improved transportation enabled more and more western farmers to escape a self-sufficientway of life and enter a national market economy during periods when commodity prices werehigh, the rate of westward migration increased spectacularly. “Old America seemed to bebreaking up and moving westward,” observed an English visitor in 1817, during the first greatwave of migration. Emigration to the west reached a peak in the 1830’s, whereas in 1810 onlya seventy of the American people lived west of the Appalachian mountains, by 1840 more thana third lived there.
What can be inferred from paragraph 1 about western farmers prior to 1815?
○They had limited their crop production to wheat, corn, tobacco, and cotton.
○They were able to sell their produce at high prices.
○They had not been successful in raising cattle.
○They did not operate in a national market economy.
对应关键句：After 1815 improved transportation enabled more and more western farmersto escape a self-sufficient way of life and enter a national market economy during periodswhen commodity prices were high, the rate of westward migration increased spectacularly.
注意原文中出现的表示对比的关系词：by contrast, unlike, while, whereas,on the contrary, incomparison to, distinct from, different from等词。
Paragraph 1: It should be obvious that cetaceans-whales, porpoises, and dolphins-aremammals. They breathe through lungs, not through gills, and give birth to live young. Theirstreamlined bodies, the absence of hind legs, and the presence of a fluke and blowholecannot disguise their affinities with land-dwelling mammals. However, unlike the cases of seaotters and pinnipeds (seals, sea lions, and walruses, whose limbs are functional both on landand at sea), it is not easy to envision what the first whales looked like. Extinct but, already fullymarine cetaceans are known from the fossil record. How was the gap between a walkingmammal and a swimming whale bridged? Missing until recently were fossils clearlyintermediate, or transitional, between land mammals and cetaceans.
Which of the following can be inferred from paragraph 1 about early sea otters?
○It is not difficult to imagine what they looked like
○There were great numbers of them.
○They lived in the sea only.
○They did not leave many fossil remains.
对应关键句：However, unlike the cases of sea otters and pinnipeds (seals, sea lions, andwalruses, whose limbs are functional both on land and at sea), it is not easy to envision whatthe first whales looked like.