GWD 4-4

A new hair-growing drug is being sold for three times the price, per milligram, as the drug’s maker charges for another product with the same active ingredient.

我觉得C是正确的

A.B，as前后结构不对称

C.that引导后面的定语从句，其中的per milligram可以看作是插入的解释成分

D.同意楼主的说法

E.我觉得charge for sb/sth of更好一些吧，但不能确定，反正D 很别扭，再有就是它也没有C 简洁啊

也就是说限定性从句和先行词之前可以插入成分，而且用逗号隔开？有"," 为什么不用which做引导词？

好，找到了一个好的解释：http://forum.chasedream.com/dispbbs.asp?BoardID=23&replyID=642101&id=72871&skin=0

同意C.

that 引导的是同位语从句, 后面的从句用来解释price。

D: 如果what 指代price，则出现price of price。但price of sth.中的sth需要是另一个对象，如price of the watch手表的价格。

E: at which 中的at似乎不妥。如果看作定语从句的话，变成charge for sth. at (price)。应该是charge (price) for sth. 如the maker charges 30RMB for the drug.

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上面链接中大量的帖子认为是限定性定语从句，找语法书，再读之后，可以比较肯定地说这里是同位语从句，因为 that the drug's maker charges for another product 是一个确定的数字，也就是等同一个price。

如果that后的句子为 that is very high等之类的内容，则没有具体的数字对应，那么从句只能做定语，而不是同位语。

OG里有一个同位语从句的用法：

OG10-194. Presenters at the seminar, one of whom is blind, will demonstrate adaptive equipment that allows visually impaired people to use computers.

另外，that引导的限定性定语从句之前也是可以有逗号的，例如OG10-8，正确答案D：People tend to overestimate the amount of energy used by visible equipment, such as lights, that must be turned on and off. 与上面的that从句比较，能体会到这两个that从句的区别。

"上面链接中大量的帖子认为是限定性定语从句，找语法书，再读之后，可以比较肯定地说这里是同位语从句，因为 that the drug's maker charges for another product 是一个确定的数字，也就是等同一个price。"

Totally disagree.

如果是apppositive clause. That 省略以后 不影响句子理解。The drug's maker charges for another product怎么理解成a price.

另外原句中the price,前的定冠词 the就是that限定性定语从句的标志。

谢谢质疑！让我加深了对同位语从句的理解。

不过你说The 是限定性定语从句的标志就不带普遍性，薄冰语法中大量同位语从句的先行词前都有the：Early in the day came the news that Germany had declared war on Russia.

所以判断标准不是看the这个冠词，而是看从句是否和先行词对等，如这个例子中Germany had declared war 本身就是一条News，相比上贴，我对price那个理解确实有误。再谢。

就用你上面说的“That 省略以后 （影响）不影响句子理解。”这个方法应该就行了，有影响的，说明从句内容关键，那就是限定性定语从句

which引导起辅助解释作用，删除不影响理解的从句就是非限定性的

对等关系就是同位语从句

18.Computers are becoming faster, more powerful, and more reliable, and so too are modems, they are the devices to allow two or more computers to share information over regular telephone lines.

(A) so too are modems, they are the devices to allow

(B) so too are modems, the devices that allow

(C) so too modems, the devices allowing

(D) also modems, they are the devices that allow

(E) also modems, which are the devices to allow

237. It seems likely that a number of astronomical phenomena, such as the formation of planetary nebulas, may be caused by the interaction where two stars orbit each other at close range.

(A)  may be caused by the interaction where two stars orbit each other

(B)  may be caused by the interaction between two stars that each orbit the other

(C)  are because of the interaction between two stars that orbit each other

(D)  are caused by the interaction of two stars where each is orbiting the other

(E)   are caused by the interaction of two stars orbiting each other

Choice C can be faulted because to form a passive construction, are should take a verb form such as caused rather than an adverb such as because.
        Also, the phrase two stars that orbit each other
illogically suggests that there are two particular stars causing all
the phenomena in question, rather than various sets of stars in various
locations.