Mountaineers carry oxygen cylinders with them because they need it for

Correct option is C

Oxygen kit is most important for a mountaineer to carry. Mountaineers carry oxygen cylinder while climbing high mountains to avoid suffocation. As we go higher and higher, at high altitudes, amount of oxygen present in the atmosphere is determined by atmospheric pressure. Though the concentration of oxygen is 21% and is constant throughout the air, the layer of air at heights above than 12000 feet becomes thinner and thinner. This decreases the number of oxygen molecules per breath making the person suffocated at high altitudes.