请教Prep2－56， 关于分词短语的疑问

56.       (27418-!-item-!-188;#058&003510) 考点：比较，逻辑表达

Unlike crested wheatgrass, an alien species from Siberia that forms only shallow roots and produces tall above-ground shoots, native North American grasses develop an extensive root system, allowing for greater enrichment of the soil, but do not grow to be very tall.

(A) native North American grasses develop an extensive root system, allowing for greater enrichment of the soil, but do not grow to be very

(B) native North American grasses develop an extensive root system, which allows for their greater enrichment of the soil, but without growing to be that

(C) the root system of native North American grasses is extensive, allowing for greater enrichment of the soil, but they do not grow to be very

(D) the root system of native North American grasses is more extensive, allowing for the greater enrichment of the soil, but these grasses do not grow to be very

(E) the extensiveness of the root system of native North American grasses allows for their greater enrichment of the soil, but they do not grow to be that

――这道题选A，我的疑问是难道这里的分词短语allowing for 没有既可以做定语，也可以做状语的歧义吗？

请教做这类题怎么进行判断什么时候有歧义，什么时候没有歧义哪。

一直对n+分词短语,这类的题琢磨不透，有的时候说分词有歧义，有的时候反而又可以是正确答案。

谢谢windfalls MM

原来我一直以为，“,doing”结构是做可以做定语也可以作状语，但是经过Asrgbf先生的指点。

我开始认为",doing”在之前有过S+V+O的前提下，最好把它当成 是 “伴随状语”，这种伴随是一种自然而然的，顺理成章的结果，当然也有可能是方式。

具体来说：

这道题目里：当native North American grasses develop an extensive root system。自然的结果就是allowing for greater enrichment of the soil，另外最好不要 把 allowing for

greater enrichment of the soil 看成是修饰 某个 动词的，而是自然 表现 前句内容形成的方式和结果。

如果修饰root system，我觉得就没必要把 ",doing"这种结构放在句尾了。“，doing"是优先做伴随状语的。

如果要修饰root system,改成从句就好了 root system that allows for their greater enrichment of the soil.

另外，即使 ”doing“之前，没有逗号，适当的时候 "doing”结构在gmat里也会被看成伴随状语。

Scientists believe that unlike the males of most species of moth, the male whistling moths of Nambung, Australia, call female moths to them by the use of acoustical signals, but not olfactory ones, and they attract their mates during the day, rather than at night.

(A) by the use of acoustical signals, but not olfactory ones, and they attract

(B) by the use of acoustical signals instead of using olfactory ones, and attracting

(C) by using acoustical signals, not using olfactory ones, and by attracting

(D) using acoustical signals, rather than olfactory ones, and attract

(E) using acoustical signals, but not olfactory ones, and attracting