(i) low temperature

(ii) low atmospheric pressure

(iii) high atmospheric pressure

(iv) both low temperature and high atmospheric pressure

Correct Answer: (ii) low atmospheric pressure

Explanation: Low oxygen concentration in the blood and tissues of people living at high altitudes is due to low atmospheric pressure. Because at very high altitudes, the oxygen component is less than that at ground level. This decrease in atmospheric pressure causes the loss of oxygen in the blood.