Sentence Correction

(A) either the math department chair or the members of the department post a challenge problem to the student body, but usually it is so difficult that they
(B) either the math department chair or the members of the department post a challenge problem to the student body, but usually the problem is so difficult that the students - correct
(C) either the math department chair or the members of the department posts a challenge problem to the student body, but usually the problem is so difficult that they
(D) either the members of the math department or the department chair posts a challenge problem to the student body, but usually it is so difficult that they
(E) either the members of the math department or the department chair post a challenge problem to the student body, but usually the problem is so difficult that the students

First let us look at the verb 'posts' as the word posts refer to the subject that is coming after 'or' and the subject is plural we need to change it to post option C is down.

Now let us check the second part of the sentence, the reference of the pronoun 'it' is ambiguous, let's put word 'problem' in the place of pronoun. This knocks down option A and D.

Now the reference of plural pronoun 'they' is unclear too. Replace it with students. So the right must be option (B)


Any other ideas?