Dental caries and gingivitis can be exacerbated not only by the foods

Dental caries and gingivitis can be exacerbated not only by the foods patients eat but also by when the patients eat them.

(A) not only by the foods patients eat but also by when the patients eat them
(B) by not only the foods patients eat but also by when the patients eat them
(C) not only by the foods patients eat but also by time when the foods are eaten
(D) by not only the foods that are eaten by patients but also by the times the foods are eaten
(E) not only by what patients eat but also by when they eat it

Answer E.

There is a problem with parallelism in the original sentence. "...not only by the foods but also by when" is not parallel. If the phrase begins by comparing food, which is a noun, the second half of the comparison must also be a noun. As it is currently constructed, "by the foods" is compared to "by when the patients eat". In order to correct the error, select answer E. "By what patients eat but also by when they eat it" maintains parallelism in two ways. First, what and when are both parallel. Second, both words are preceeded by the word by.