Read this passage.
I'd figured my manuscript would land in the slush pile with all
of the others that the publisher didn't solicit. I'd thought that
at best an intern would read it and email me something to the
effect of "Thanks, but no thanks." So you can imagine my
astonishment when an editor called me and told me that not
only had my manuscript been accepted, I was also going to
get an advance. That early paycheck allowed me to focus on
revisions (and there were plenty of them!) without worrying
about how I would pay rent.
The day my galleys arrived was surreal. I combed through the
pages of words-my words-realizing that my book would
soon be born. And even though the font wasn't final and the
illustrations weren't included, I felt like I had finally become a
real author.

What does the phrase slush pile mean in the passage?

A collection of submissions that a publisher did not
request

B stack of documents set aside specifically for an
intern

C group of submissions that a publisher deemed
inferior

D Batch of submissions that a publisher has rejected