16. Not woman held a presidential cabinet position in the United States until 1933, when Frances Perkins became secretary of labor.
答案：A
测试点：否定词。
分析：no，not以及none这三个否定词的用法，常在题中故意混淆。副词not不能修饰名词woman，此外应用形容词no。
解题要点：no或not出现在TOEFL改错题中，应首先考虑其是否正确。
17. The human body relies on certainty nutrients for its survival.
答案：C
测试点：词性。
分析：此处修饰名词nutrients要用形容词certain，不用名词certainty.
18. Too much electric current may flow into a circuit as a result either of a fault in the circuit and of an outside event such as lightning.
答案：D
测试点：连词短语
分析：either…or…的用法。看到either这个词，要注意寻找其后是否有相应的or。and应为or。
解题要点;either…or，neither…nor, both…and等连词习语为TOEFL常考题型，注意其前后搭配。
19. The Appalachian Trail, extending approximately 2,020 miles from Maine to Georgia, is the longer continuous marked footpath in the world.
答案：C
测试点：最高级。
分析：形容词或副词前有the，后有in the world等表示范围—类的短语，应当用最高级形式。此处longer应为longest。
解题要点;当TOEFL改错题中出现划线的形容词或副词的比较级或最高级形式时，应注意其用法是否正确。
20. For years, elephants were hunted for food and ivory, and as a result theirs numbers have been greatly reduced.
答案：B
测试点：词性。
分析：numbers为名词，前面要用形容词性的代词their。theirs为名词性质
