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:

10% of the air molecules in the atmosphere
50% 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
A food web is

A set of interconnected food chains, illustrating the tropic relationships among species
The tropic (feeding) levels that can be observed in an ecosystem
Like a food chain
The set of prey species that are consumed by animals in an ecosystem

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