It means the more meat there is then, corn products will start to go down, but if it was the other way aroudn then it would be, corn goign up and meat going down which is what its telling you.
We know it's not A.) because that's not what they are talking about. We know it can't be B.) because the author is not persuading us (the readers) to do anything. Between C.) and D.) the only one that makes sense is D.