Cause it says always in the stimulus so in order to weaken the argument you just have to provide one example where an extra layer wouldn't be sufficient to keep your hands warm. Which is what C does.
Also The stimulus doesn't say the reason extra layers makes your hands warmer is because of the increased blood circulation from the activity of putting them on, rather it says it's because the extra layers warm your vital organs. So it's not really attacking any argument made in the stimulus.