Why water boils at a lower temperature at higher altitudes?

Q. Water boils at a lower temperature on the hills because

1. there is less carbon dioxide on the hills
2. it is cold on the hills
3. there is a decrease in air pressure on the hills
4. There is less oxygen

Answer: C

Water boils at a lower temperature on the hills because there is a decrease in air pressure on the hills.

The boiling point of water is lower at higher altitudes due to the decreased atmospheric pressure. High-altitude cooking requires a compensation for lower temperatures for any cooking that involves boiling or steaming. This effect starts to become relevant at altitudes above 2,000 feet (610 m). Means of compensation include boiling/steaming for longer times or using a pressure cooker to provide higher pressure inside the cooking vessel and hence higher temperature.

At sea level water boils at 100°C. For every 500 ft (~150 m) increase in elevation, the boiling point is lowered by 0.5°C. For 8,000 ft (~2500 m) elevation, water boils at 92°C. Boiling as a cooking method must be adjusted or alternatives applied. Vegetables and some starches will simply take longer to cook while rice and legumes (beans) need a pressure cooker. Pasta will need a pressure cooker.