GMAT Sentence Correction (SC) | Re: OG 12th Edition: Q81. Recently physicians have determined

OG 12th Edition:

Q81. Recently physicians have determined that stomach
ulcers arenot caused by stress, alcohol, or rich foods,
but a 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 areby


Can someone please explain why you are not allowed to insert an 'are' in this idiom? (answer choice (B))