GMAT  Official Guide是每一个考GMAT的学生必须具备的书籍。里面的每一道题目最好做2遍以上（尤其是错题，一定要好好分析体会）。很多同学告诉我，OG的答案不够清楚。事实上，OG的有些讲解试图绕开一些需要补充的知识，例如什么是溯因推理，分词的用法等等，因此往往显得不够清晰。因此，我想我每天抽出一些时间，写一道OG的题目，帮助大家学习。

11. (难度600)Travelers from Earth to Mars would have to endure low levels of gravity for long periods of time, avoiding large doses of radiation, plus contending with the chemically reactive Martian soil, and perhaps even ward off contamination by Martian life-forms.
A. Mars would have to endure low levels of gravity for long periods of time, avoiding large
doses of radiation, plus contending
B. Mars would have to endure low levels of gravity for long periods of time, avoid large
doses of radiation, contend
C. Mars, having to endure low levels of gravity for long periods of time, would also have
to avoid large doses of radiation, plus contending
D. Mars, having to endure low levels of gravity for long periods of time, avoid large doses
of radiation, plus contend
E. Mars, who would have to endure low levels of gravity for long periods of time, avoid
large doses of radiation, contend with

比较简单的题目，看懂以后很快就发现了平行对称结构：
Have to (1) endure low levels of gravity…, (2) avoiding large…, (3) plus contending with …, and (4) perhaps even ward off…
注意（4）是没有划线的，也就是说不能更改的，因此前面必须都是省略to的不定式，由此可以排除A, C

而D,E大改原句的结构，本来都是have to(不得不做)的，被修改后have to 只跟endure一个动作，改变原文的合理意思。

答案：B