This is a weaken question, so we need some new information in an answer choice that makes the argument worse. The argument essentially concludes that consumers can make a significant long-term reduction in unrecycled waste by not purchasing plastic with higher numbers, since higher numbers are less likely to be recycled. (C) tells us that recycled plastic containers are the ones that tend to have the higher numbers. But this means that consumers won't be buying recycled plastic, which means that there won't be incentives to produce recycled plastic, and so this could actually hurt recycling in the long run. So (C) helps weaken the argument.
(E), on the other hand, tells us only that communities that collect all discarded plastic containers for potential recycling only dump high-numbered plastics when it's clear that no recycler will take them. But this doesn't necessarily help solve the problem, since we're told in the stimulus it's more likely that no recycler takes them anyway. Also, those might be a minority of communities, and this question is talking about the actions of consumers in general. Thus, it's not as effective a weakener as (C). Hope this helps!
