The paper is well organized and presents a worthwhile and interesting idea. This idea is supported by sound evidence presented in a neat and orderly way in accordance with recognized conventions for citing evidence. Some of the sentences may be unwieldy now and then, but they are organized around one main idea. The reader does not have to read a paragraph two or three times to get the thought that the writer is trying to convey. The thesis in the C paper frequently hangs on some personal opinion.
Opinion is often the engine that drives an argument, but opinion by itself is never sufficient.
The writer must defend an opinion in a persuasive fashion. Even if it has a clear and interesting thesis, a paper with insufficient supporting evidence is a C paper. The C paper often has mechanical faults and errors in grammar and spelling. Please note that a paper without such faults may still be a C paper. A paper with such faults, however, will never be more than a C paper.