The underlined part of sentence 3 <em>that feeds the bottom dwellers </em>is used as a noun clause, because it is a part of the noun phrase <em>constant rain of organic matter.</em>
They probably did this because the idea of a real spider causes the story to be more believable in the sense of how he turns into “The Spider Man”. If it was a computer generated “spider”, it wouldn’t real my be a spider meaning that he wouldn’t really be a “spider - man”.
C? I believe... That's a toughy!