Mostly b.)

But I also think perceptions change this time of year. It's dark more frequently, so you're getting less visual input from the periphery. It's harder to see trees, bushes and telephone poles whooshing by because it's dark out. You develop a sort of tunnel vision and pedal harder because you think you're going slower (even though you're not), and that tires you out.

But mostly winter clothes, especially from the waist down. I try to wear shorts no matter how cold it gets.