SC - Who vs. Whom

[ArjunJ720]

Hi,

Upon reviewing my CAT exams , I came across this sentence correction question that I got wrong.

Elderly patients, many who take expensive brand-name medications daily, could reduce their medical costs by switching to generic drugs and making lifestyle changes.

(A) many who
(B) many of them who
(C) and many of them who
(D) many of whom
(E) many of which

The correct answer is (D). I looked up when to use who vs. whom and found out that who should refer to the subject and whom should refer to the object. My question is, how do I know that patients is the object by reading this sentence?

Thanks.

[Chelsey Cooley]

This was posted twice; I answered the question in the other thread here: https://www.manhattanprep.com/gmat/foru ... 32392.html