Please choose the correct answer to the question: Why do mountain climbers at high elevations use oxygen tanks to help them breathe?

Answer1: At high elevations the ozone layer draws oxygen out of the atmosphere. Answer2: The atmosphere is less dense at higher elevations so there is less oxygen available. Answer3: Oxygen is heavier than the other gases in the atmosphere and sinks to lower elevations. Answer4: Radiation from the Sun splits oxygen molecules into atoms making the oxygen unbreathable.

Answer format: answer1/answer2/answer3/answer4