A colorless, odorless gas from incomplete combustion that blocks oxygen transport in blood.

• A. Ground-level ozone
• B. Carbon monoxide
• C. PM2.5
• D. Nitrogen oxides

Answer: (B)

The main idea is how a gas produced by incomplete combustion can impair oxygen delivery in the body. Carbon monoxide fits this description: it’s colorless and odorless, produced when fuels burn without enough oxygen. Once inhaled, CO binds to hemoglobin in red blood cells far more readily than oxygen, forming carboxyhemoglobin. This blocks red blood cells from carrying as much oxygen to tissues, leading to tissue hypoxia and potentially serious harm even at low levels. The other options don’t disrupt oxygen transport in the blood in this specific way. Ground-level ozone is a reactive pollutant that irritates the airways but doesn’t bind to hemoglobin. PM2.5 are fine particles that cause health effects through deposition in the lungs and systemic inflammation rather than a direct blockade of oxygen transport. Nitrogen oxides are gases that contribute to smog and respiratory irritation but do not bind oxygen in the blood. So the gas that best matches the description is carbon monoxide.