A Sudden or Total Lack of Oxygen Can Cause Unconsciousness

A sudden or total lack of oxygen can cause unconsciousness in 5-10 seconds. Why?

A) The brain has very high energy requirements and lack of oxygen reduces available energy.
B) Oxygen is required for ventilation and lack of ventilation causes fainting.
C) Lack of oxygen causes an increase in carbon dioxide, which is toxic.
D) The brain needs oxygen to breathe, and when oxygen is not available, it signals the nervous system to cause fainting.