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.
- not caused by stress, alcohol, or rich foods, but
- not caused by stress, alcohol, or rich foods, but are by
- caused not by stress, alcohol, or rich foods, but by
- caused not by stress, alcohol, and rich foods, but
- 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. | 
