I totally can see why C would be the right answer.
However, I have to say in reading everyone's questions and the rational given for why the other four are incorrect, it seems that this particular weaken question is really more about the best answer than just four "not weaken" and one correct answer. Would you say that is right?
I keep looking to eliminate four answers that either don't weaken as in go along with the conclusion or do nothing to the conclusion. When I read, for example, answer D, I consider that the author's conclusion that people should purchase plastics with labels in the lower number category, that this effort would be sabotaged by the idea that higher cost would deter people from doing so. This
could undermine the author's effort in the conclusion. Obviously, not the best answer as compared to C
.and yet I am curious if the fact that cost is the deterrent, does it matter because there are people who would be willing to pay more?