Here's a problem to solve: On cold days, snakes usually lie very still and eat little or nothing, while birds usually move around and eat a lot of food. Which statement best explains this?

Among the 4 following options, which is the correct answer?

- A: Both animals are cold-blooded, but without feathers to keep warm, snakes get too cold to move.
 
- B: Unlike birds, snakes are warm-blooded; they must hibernate during cold weather.
 
- C: Unlike snakes, birds are cold-blooded; they are less affected by the cold than snakes.
 
- D: Unlike snakes, birds are warm-blooded; they must eat food to maintain a constant temperature.
 