Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Why do mountain climbers at high elevations use oxygen tanks to help them breathe?
The atmosphere is less dense at higher elevations so there is less oxygen available.
To supplement the decreased oxygen levels at high altitudes.