Yeah but C makes more sense. 0-10 is cold but not freezing, 10-20 is cool, 20-30 is warm, 30-40 is hot, 40+ is "you're gonna die of heat exposure! Get inside, what are you doing?!" increasing in urgency with the number. If it's in the negatives, it's the same as the 40+ except "cold exposure".
I wouldn’t say it corresponds between negative and positive. -20 is already a lot more dangerous than 40.
Depends where you live, I guess. I don't really consider -20 to be dangerous but I live in Canada so I know how to dress for cold weather.

I consider any place you can’t sleep outside on the ground and wake up the next morning, uninhabitable.

cold bad. human water, like fluid state not ice state