You Get Goose Bumps When You're Cold

You get goose bumps when you're cold. What purpose does that serve?

A) They thicken the skin, improving insulation.
B) Nothing really. It's just a reflex.
C) They puff up fur to trap air that helps retain body heat for mammals that have fur or hair on their skin.
D) They are a shivering mechanism that helps increase metabolic rate to increase heat production.
E) Because blood circulation is directed away from the skin when a body is cold, the skin shrinks a little bit and this shows up as goose bumps.