Why must a mountain climber at 8,000 feet use more energy to breathe in the same amount of oxygen from the atmosphere?

A. None of these answers are correct.
B. As altitude increases, the air pressure increases and there are fewer oxygen molecules in a given volume.
C. As altitude increases, the air pressure decreases and there are fewer oxygen molecules in a given volume.
D. As altitude increases, the air pressure increases and there are more oxygen molecules in a given volume.