老prep中的几个问题，盼赐教

emmatt

各位前辈， 在做老prep中，遇到几个问题，想问一下自己不太明白的地方。

9.    (24819-!-item-!-188;#058&001693)

The Environmental Protection Agencyfrequently puts mandatory controls on toxic substances that present aslittle risk as one in a million chances to cause cancer.

(A) as little risk as one in amillion chances to cause

(B) as little risk as one chance ina million of causing

(C) as little risk as one chance ina million that it will cause

(D) a risk as little as one chancein a million for causing

(E) a risk as little as one chancein a million for it to cause

答案是说risk of doing sth做为正确的习惯用法。 这点我清楚了。
请问： D选项，如果变为 a risk as little as one chance in a million of causing， 这样可以算对吗？ 、

8.    (24773-!-item-!-188;#058&001691)

Ozone, a special form of oxygenthat screens out harmful ultraviolet rays, reaches high concentrations twelvemiles above Earth, where it has long appeared that it was immune fromhuman influence; we have now realized, though, that emissions of industrialchlorofluorocarbons deplete the ozone layer.

(A) has long appeared that it wasimmune from

(B) has long appeared to have beenimmune from

(C) has long appeared as beingimmune to

(D) had long appeared immune to

(E) had long appeared that it wasimmune to

答案D，为什么要用过去完成时态呢？给出的解释是与分号后的现在完成时形成对比，那如果用“appeared immune to", 对吗？ 一定要用过去完成时的理由是什么呢？  

25.  (25704-!-item-!-188;#058&002607)

There is a widespread belief in theUnited States and Western Europe that young people have a smaller commitmentto work and a career than their parents and grandparents and that thesource of the change lies in the collapse of the "work ethic."

(A) a smaller commitment to workand a career than their parents and grandparents

(B) less of a commitment to workand a career than their parents and grandparents

(C) a smaller commitment to workand a career than that of their parents and grandparents

(D) less of a commitment to workand a career than their parents and grandparents had

(E)a lessening of the commitment to work and a career that their parents andgrandparents had

我知道在than/as的比较中，尽量补出谓语动词，than/as 之后用do，does，did，is, was, were, are。 我理解此处的have是实义动词，为什么答案D中，不是“ less of a commitment to work and a career than their parents and grandparents did”， 而要用had呢？ 请诸位赐教。

谢谢诸位的时间。

enkyklios

果然很老的题

zxppx

这里有第三题manhattan的解释
http://www.manhattangmat.com/forums/there-is-a-widespread-belief-in-the-us-and-western-t3636.html


there is ambiguity if you eliminate the helping verb.
without the helping verb, there are two interpretations:
1: young people are less committed to work/career than WERE their parents/grandparents (the intended meaning)
2: young people are less committed to work/career than TO their parents/grandparents
you can't tell which of these is the correct meaning without the helping verb; therefore, the helping verb is necessary.

尽管有个人提出了和你一样的问题，
Sentence 1: I have more toys than you [do]. -- do is avoidable here.
Sentence 2: I have seen more toys than you have.
但是我认为Ron解释得不清楚

emmatt

我理解do这个助动词是一定要补出拉。。省略是肯定不对的。 只是不明白为什么不用Did，而用had。

看下来，应该是是要平衡的关系。 had能更清楚的表达，也前面have对称，其实用了did，也不算错，是这么个意思吗？