I believe it is Nerve Cells
Correct me if I'm wrong.
D, light can travel in the air or a vacuum and the speed is constant whether the light source is stationary or moving.
Answer: .D. Smaller because its smaller surface area to volume ratio would allow it to breathe sufficiently through its skin alone.
Explanation:
The two differences you see when comparing the four chambers of the heart is that the arteries have a thicker wall and veins have thin walls and that the arteries carry oxygenated blood away from the heart which means they get more pressure than veins.