21. Barges which carrier most of the heavy freight on rivers and canals are usually propelled by towing.
答案：A
测试点：词性。
分析：carrier是名词，而此处需要谓语动词。应为carries。
解题要点：名词后缀-er,-or,-tion,-sion等形式出现在TOEFL改错题中，应引起特别注意。
22. Although afflicted by serious eyesight problems, Alicia Alonso was one the principal stars of the American Ballet Theater and later formed her own dance company.
答案：C
测试点：习语。
分析：One of the+复数名词/名词词级是固定说法，此处缺of.
23. The ritual combat of animals are triggered by precise signals.
答案：B
测试点：主谓数的一致。
分析：主语是单数的combat,系动词要用心
解题要点：介词短语of animals是名词combat的定语，这类以of引接的定语后置结构要特别注意主谓的数是否一致。
24. It is more difficult to write simply, directly, and effective than to employ flowery but vague expressions that only obscure one’s meaning.
答案：B
测试点：并列结构/词性。
分析：and的前后部分应同词性、同性质，此处应用副词effectively与and前的simply和directly并列，不用形容词effective。
25. Different species of octopuses may measure anywhere from two inches to over thirty feet in long.
答案：D
测试点：词性
分析：表示“在长度方面”固定的说法是in length，不用long。从词性考虑，介词诅后要加名词(1ength),不能加形容词(1ong)。
解题要点;TOEFL考题中表示长(1ong，length),高(high，height),深(deep，depth)等词的名词及其形容词形式常常混淆。
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