a grammar question

toefl_2003

37. A common way to produce x-rays is _________ electrons into a copper target.
   A fires    B fired    C by firing    D how to fire   

I find that each answer sound not good. Though I choose C, which is the right answer, I don't know why.  If someone knows the rule, could you tell me, please? If you can give me another example, it will be better.

Trunnity

我认为选C， 表方式。 也可以用排除法。A是动词，显然不可以，B，被动语态后不能接宾语，所以也不对，D, 语义不对。

呵呵~~~ 个人见解~~~

困难是弹簧

Trunnity是个牛MM

Trunnity

以下是引用困难是弹簧在2003-6-19 9:41:00的发言：
Trunnity是个牛MM

那会不会很胖啊~~~

困难是弹簧

牛MM都不胖的说