24. Because of their rapidly changing economically fortunes, many frontier towns of the American West underwent spectacular fluctuations in population in the nineteenth century.
the answer is C, why?
34.European settlers in North America moved from the Atlantic coast across 3,000 miles forests, grasslands, deserts, and mountains until they reached the Pacific Ocean.
C. 是不是应该为miles of forests?
36.In this world of high technology ,it is easy to forget that the most important tools ever developed for learning is still the book..
the answer is C, why?
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