Hi could an instructor explain the issues with choice C. I think the only issue is the reference for the pronoun "it". Also what is the difference between choice A and B. Is it only that as well as cannot be used to form a compound noun? Would really appreciate an explanation. Thank in advance.

This thread provides a nice explanation of why choice C isn't so hot.

Please try to search the forum for relevant threads before posting: this thread was the first hit under 'comet' (use the search box in the upper right-hand corner of forum pages).