Environmental Science
Much of Tibet lies at altitudes over 18,000 feet where the pressure is about 500 mb. At such altitudes, the Tibetans are above roughly:

50% of the air molecules in the atmosphere
10% of the air molecules in the atmosphere
75% of the air molecules in the atmosphere
25% of the air molecules in the atmosphere

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Environmental Science
Definition of ecosystem is

The abiotic component of a habitat
The community of organisms together with the environment in which they live
The part of the earth and its atmosphere which inhibits living organisms
A community of organisms interacting with one another

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