Asphyxiation can be caused by which of the following?

Answer: B — Breathing toxic substances

Asphyxiation occurs when the body does not receive sufficient oxygen to sustain normal physiological functions, which can lead to various health complications or death. Breathing toxic substances is a primary cause of asphyxiation because these substances can interfere with the body's ability to use oxygen effectively or directly harm the respiratory system. For instance, inhaling carbon monoxide prevents oxygen from binding effectively to hemoglobin in the blood, which is essential for breathing and delivering oxygen to tissues. While low oxygen levels can also lead to asphyxiation, the scenario specified in the question highlights "breathing toxic substances" as a direct and immediate cause. Toxic substances can cause rapid hypoxia or impede the oxygenation process directly in ways that low oxygen levels might not, making breathing toxic substances a critical focus when discussing asphyxiation. The other options deal with different physiological effects or situations that may not directly lead to asphyxiation in the same manner.