Q19:

Recently physicians have determined that stomach ulcers are not caused by stress, alcohol, or rich foods, but a bacterium that dwells in the mucous lining of the stomach.

1. not caused by stress, alcohol, or rich foods, but
2. not caused by stress, alcohol, or rich foods, but are by
3. caused not by stress, alcohol, or rich foods, but by
4. caused not by stress, alcohol, and rich foods, but
5. caused not by stress, alcohol, and rich foods, but are by

这道题答案是C, 哪位nn讲讲为什么by不可省略? 为什么not要在caused 的后面?

多谢.

平行结构：

not by x, but by y