84. Recently physicians have determined that stomach ulcers arenot caused by stress, alcohol, or rich foods, buta bacterium that dwells in the mucous lining of the stomach.
(A) not caused by stress, alcohol, or rich foods, but
(B) not caused by stress, alcohol, or rich foods, but are by
(C) caused not by stress, alcohol, or rich foods, but by
(D) caused not by stress, alcohol, and rich foods, but
(E) caused not by stress, alcohol, and rich foods, but are by
正确答案为C
og原版解答节选如下：
A To preserve parallelism,butshould be followed byby.
B There is no reason to repeat the auxiliary verbare.
想问一下,A,B选项中，but 后不用加caused by吗？只加by 就可以了吗？为什么caused可以省略？
希望大神可以给予解答，谢谢！ | 
