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　　  (A) Discussion of greenhouse effects have usually hadas a focus the possibility of Earth growing warmer and to what extent it might

　　  (B) Discussion of greenhouse effects has usually hadas its focus whether Earth would get warmer and what the extent would be

　　  (C) Discussion of greenhouse effects has usually focused on whether Earth would grow warmer and to what extent

　　  (D) The discussion of greenhouse effects have usually focused on the possibility of Earth getting warmer and to what extent itmight

　　  (E) The discussion of greenhouse effects had usually focused on whether Earth would grow warmer and the extent that is

　　  这道题的正确答案是C。很多同学没有选C，因为找不到C中的 to what extent 更谁平行。这里的 to what extent 不是不定式的用法，它相当于是一个从句的引导词，往前找可以和 whether 平行。To what extent 这个用法我们可以在哪里学到呢?我们可以在另一道题的错误选项中看到这个表达。来我们看一下下面这个题↓↓↓