关于这个问题，OG里面最权威的解释是来自这个题：
243. Some scientists have been critical of the laboratory tests conducted by the Federal Drug Administration on the grounds that the amounts of suspected carcinogens fed to animals far exceeds those that humans could consume.
(A) far exceeds those that humans could consume
(B) exceeds by far those humans can consume
(C) far exceeds those humans are able to consume
(D) exceed by far those able to be consumed by humans（E）
(E) far exceed those that humans could consume
In D, the use of the passive voice in the phrase those able to be consumed by humans is unjustified, as it increases wordiness while stating the meaning less precisely: it is accurate to call humans “able,” but not to call those [amounts] “able.”
用able to be在大全里搜索，几乎所有的题目考察点和正确选项的特点都惊人的一致，感觉ETS好像认为be able to是一个主动性很强的词，该词的逻辑主语必须具有发出be able to 后动词的能力，如果是该动作的承受方则一般都改成了can/could。看完下面若干例子，相信以后不会再上ETS的当了，呵呵。
(A) to see whether pigeons can be trained to help find
(B) to see whether pigeons can be trained as help to find
(C) to see if pigeons can be trained for helping to find
(D) that see if pigeons are able to be trained in helping to find（A）
(E) that see whether pigeons are able to be trained for help in finding
10. Climatic shifts are so gradual as to be indistinguishable at first from ordinary fluctuations in the weather.
(A) so gradual as to be indistinguishable
(B) so gradual they can be indistinguishable
(C) so gradual that they are unable to be distinguished
(D) gradual enough not to be distinguishable（A）
(E) gradual enough so that one cannot distinguish them
6. There is ample evidence, derived from the lore of traditional folk medicine, that naturally occurring antibiotics are usually able to be modified to make them a more effective drug.
(A) are usually able to be modified to make them a more effective drug.
(B) are usually able to be modified to make them more effective drugs
(C) are usually able to be modified, which makes them more effective drugs
(D) can usually be modified to make them a more effective drug（E）
(E) can usually be modified to make them more effective drugs
10. When Congress reconvenes, some newly elected members from rural states will try and establish tighter restrictions for the amount of grain farmers are to be allowed to grow and to encourage more aggressive sales of United States farm products overseas.
(A) and establish tighter restrictions for the amount of grain farmers are to be allowed to grow and to encourage
(B) and establish tighter restrictions on the amount of grain able to be grown by farmers and encouraging
(C) establishing tighter restrictions for the amount of grain farmers are allowed to grow and to encourage
(D) to establish tighter restrictions on the amount of grain capable of being grown by farmers and encouraging（E）
(E) to establish tighter restrictions on the amount of grain farmers will be allowed to grow and to encourage
210. If additional deposits of oil are found, it will expand the amount that can be used as fuel and reduce the price of oil, even if the deposits are not immediately tapped.
(A) it will expand the amount that can be used as fuel and reduce the price of oil
(B) the amount that is able to be used as fuel will expand and the price of oil be reduced
(C) it will cause an increase in the amount that is able to be used as fuel and a reduction in the price of oil
(D) the amount that can be used as fuel will increase and the price of oil will drop（D）
(E) it will increase the amount of oil that can be used as fuel and cause a drop in the price
正确的用法：主动态
985 Those who have visited the Grand Canyon have typically seen layers of sediment in the gaping canyon, with different colors that mark the passage of time like the rings in a tree trunk.
(A) seen layers of sediment in the gaping canyon, with different colors that mark
(B) see layers of sediment in the gaping canyon, whose different colors mark
(C) been seeing layers of sediment in the gaping canyon, whose different colors are markers of
(D) been able to see layers of sediment in the gaping canyon, with different colors marking（D）
(E) seen layers of sediment in the gaping canyon, marking by different colo
3. Because the financial review covered only the last few fiscal years, and therefore the investigators were unable to determine the extent to possible earlier overpayments.
(A) and therefore the investigators were unable to determine the extent to possible
(B) so therefore the investigators were not capable of determining the possible extent of
(C) therefore the investigators were unable to determine the possible extent of
(D) the investigators were not capable of determining the possible extent of（E）
(E) the investigators were unable to determine the extent of possible
顶啊，解决一个大疑惑，多谢多谢
在XDF上课的时候,老师说be able to be done 永远错
剩C与D
排除C,一是时态不对,二是助动词和介词尽量补出原则来看,原文是对应think,应该补do,补出can,补错了,不选
1) computers are able to be programmed 纯粹是个逻辑错误，并不是因为新东方说了can 优于be able to
计算机有能力被编程吗？can是个情态动词，表示可以，如The plan can be implemented. 与表示有能力的be able to不一样
---- 后来再想，发现不完全是如 8 楼说的原因“该词的逻辑主语必须具有发出be able to 后动词的能力，如果是该动作的承受方则一般都改成了can/could。”错误的实质是这个主语既是be able to 的发出者又是be done的承受者，从而逻辑上有问题。即使计算机有能力编程 computers are able to program，同样不能说computers are able to be programmed.
2) assmuption 是不是决定了从句里面表示一种不确定的动作？除了做can的过去式，could还可以表示现在，用在虚拟语气从句中（我想这里就是这个用法）；或者做can的委婉形式，如：Could you do me a favor?
3) C的根本错误楼上也指出来了： “as people can省略了be programmed to think，逻辑不通，”题目并没有要对人进行编程的意思，不要将as people can 理解为as people can think 了。
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