Question

take blood back to the heart. which gas would be most abundant in a vein that takes blood from muscle cells back to the heart?
carbon dioxide
oxygen
nitrogen
hydrogen

Explanation:

Muscle cells use oxygen for cellular respiration, producing carbon dioxide as a waste product. Veins carry deoxygenated blood (high in CO₂) from tissues (like muscle cells) to the heart. So carbon dioxide is most abundant here. Oxygen is low (used by cells), nitrogen and hydrogen are not major respiratory gases in this context.

Answer:

carbon dioxide (assuming the first option is "carbon dioxide" as per the visible text)