• many vs. many of

Hi!
I read the following in a TOEIC book.

“Question 145 refers to the following letter.

To whom it may concern:

I would like to recommend Sharon Doe as a candidate for a position with your organization. …

During her tenure with ZINC company,Sharonwas responsible for supervising the department office assistants. These assistants, underSharon’s management, were responsible for 145 ______ office’s basic administrative and clerical functions.Sharoneffectively scheduled and managed several assistants to maintain efficient office operations. …

145.
A. many
B. many of
C. almost
D. many the

(Answer: A)”

I have two questions.
Q1. Do you agree that A is correct?
Q2. If so, would you explain why B is not correct?

Thanks in advance.