Yikes, I chose D thinking that younger people would be less likely to own a home and hence they would be excluded from the poll that was conducted in people's houses (too much stats class I know). I guess I completely ignored the possibility that dorms and your parent's house could still count as your house.

Yes! My guess is that this is because the assumptions are not considered "outside information", but are already pre-supposed in the argument to form a supportive relationship between the premise(s) and the conclusion. And since the argument doesn't explicitly state the assumptions, they are still available to be disputed through criticisms (sometimes based on outside knowledge), and thus weaken the support.