Environmental Science
The most abundant greenhouse gas in the earth's atmosphere:

Methane (CH4)
Chlorofluorocarbons (CFCs)
Carbon dioxide (CO2)
Water vapour (H2O)
Nitrous oxide (N2O)

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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:

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

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