疑问贴，一道知识点题目

gmat中 so as to 一般很很少连用在一起，一般使用so+adj+as+to  ,so+adj+as+to不能倒装，逻辑主语一致且一般用于很简单的主系表的结构里


其实og39和这道prep上的题我也好混乱

我先也在捉摸着一系列的题，og39的"so as to"之所以错误，是因为会有歧义，用"so that he could"就可以把意思说明白，GMAT喜欢effectiveness
（近两天版上还讨论过这题目，可以找下那个链接）
now when you use "so as to", make sure there is only one AGENT before so as to.

这有一道大全上的题，但Ron表示题目很烂。这题也是没选"so as to"项。
http://www.manhattangmat.com/forums/so-as-to-to-so-that-t8142.html

这篇Ron讲得很详细，强烈推荐
http://www.manhattangmat.com/forums/so-that-such-that-so-as-to-t13833.html


然后楼主给的这个题

Often major economicshifts are so gradual as to be indistinguishable at first from ordinaryfluctuations in the financial markets.
A.so gradual as to be indistinguishable
B.so gradual so that 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 thatone cannot distinguish them

其实主要干扰项是C，其他都很容易排除
C的问题出在  they are unable to be distinguished
以下from Ron
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i also find something a bit awkward about the use of "unable" to describe something that's not actually an inability of the subject.  

this plant is unable to perform photosynthesis.
legitimate, because this actually describes an inability of the plant itself.

this sentence:
they (= major economic shifts) are unable to be distinguished...
i don't like this, because we're not actually talking about an "inability" possessed by the economic shifts themselves.  
if we'd said something like "economic shifts are unable to destroy your equity", then i'd find that more appealing.
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其实我今天还看到Ron说一道题，错误选项有（写个大概）because of A's inability to be measured  （A是一个类似物理学上的力的东西，不记得了）
这里inability也有同样的问题......
而正确答案是because A cannot be measured

所以我们可以推而广之，很多时候be able to都不太好，换成can。一是简洁，二是在主语是non-live thing的时候，或者后面还是passive voice的时候，一般都不能用be able to。