Carbon dioxide is primarily excreted by the lungs through respiration

Steps:

• Identify the main waste product: Carbon dioxide (CO2) is produced by cellular respiration and must be removed from the body.
• Recall excretory organs: The body uses lungs, kidneys, skin, and digestive tract for waste removal, but each handles specific substances.
• Determine CO2's pathway: CO2 dissolves in blood, travels to lungs, and is exhaled as a gas during breathing.
• Compare volumes: Lungs eliminate over 99% of CO2, far more than other organs.

Why B is correct:

• Lungs facilitate gas exchange in alveoli, where CO2 diffuses from blood into air and is exhaled, per the principle of partial pressure gradients in respiration.

Why the others are wrong:

• A. Kidneys filter water-soluble wastes like urea, not gaseous CO2.
• C. Rectum eliminates solid feces, unrelated to CO2 excretion.
• D. Skin sweats water and salts, with minimal CO2 loss via diffusion.

Final answer: B