Mountaineers carry oxygen cylinders with them because:
A. Oxygen supports combustion
B. Air becomes thinner as we go higher
C. Air is denser as we go higher
D. None of these

Hint: First we should be aware of the properties of the Mountaineers which they possess because of which they were able to reach the high heights. There are many reasons given in the options, they all could be the right options but we have to take the facts which says that the amount of oxygen at height reduces due to lack in oxygen level.

Complete step-by-step answer:
Here in the given question statement the question is regarding the reason because of which the Mountaineers had to carry the oxygen cylinders along with them whenever they start the hiking of any place like mountains which is on the high altitude.
The mountaineers had to carry oxygen cylinders along with them when they started the climbing of the mountains. This is because when they climb mountains then as the altitude increases, the amount of oxygen level in the atmospheric air also decreases along with the increase. This is because the trees are not found at higher altitude because of the extreme surroundings.
As a result of this entire process, the mountaineers on their triumph suffer from dizziness. It would also lead to hypoxia in many cases along with dizziness.

Therefore we can say that the correct option among the ones would be option B, Air becomes thinner as we go higher.

Note: Unlike most sports, mountaineering lacks widely-applied formal rules, regulations, and governance; mountaineers adhere to a large variety of techniques and philosophies when climbing mountains. Numerous local alpine clubs support mountaineers by hosting resources and social activities. A federation of alpine clubs, the International Climbing and Mountaineering Federation (UIAA), is the International Olympic Committee-recognized world organization for mountaineering and climbing.