Please look at this question.

"________overseeing the direct sales end of our business, the manager will also take care of event planning."
(a)compare to (b)furthermore (c)as soon as (d)besides

I thought its correct answer seemed to be "(c) as soon as", but it is "(d)besides".

Then, Is the word of "overseeing" a noun? That's why, should it be "(d)besides"?
I thought that "overseeing" is a gerund of the verb of "oversee" so the meaning of "overseeing' was coming to me like meanings of "checking" or "supervising".

Could someone explain to me why the correct answer is "(d)besides"?
(d) is the only answer to give a grammatically correct sentence.
Thank you for your reply. In "as soon as", can't we omit a "subject"? "As soon as " is a kind of "conjunction" in my view. Thank you for another answer of yours in advance.