The correct answer is: A. 10% of the air molecules in the atmosphere
The atmosphere is divided into four layers: the troposphere, stratosphere, mesosphere, and thermosphere. The troposphere is the lowest layer and contains about 75% of the mass of the atmosphere. The stratosphere contains about 20% of the mass of the atmosphere, the mesosphere contains about 5% of the mass of the atmosphere, and the thermosphere contains less than 1% of the mass of the atmosphere.
The troposphere is where most of the weather occurs. The stratosphere is where the ozone layer is located. The mesosphere is where meteors burn up. The thermosphere is where the aurora borealis (northern lights) and aurora australis (southern lights) occur.
The atmosphere gets thinner as you go higher in altitude. At an altitude of 18,000 feet, the pressure is about 500 mb. This means that there is about 500 millibars of pressure per square inch. At sea level, the pressure is about 1,013 millibars. This means that there is about twice as much pressure at sea level as there is at an altitude of 18,000 feet.
The amount of air molecules in the atmosphere also decreases as you go higher in altitude. At an altitude of 18,000 feet, there is about 10% of the air molecules that there are at sea level. This means that there is about 10 times less air at an altitude of 18,000 feet than there is at sea level.
Therefore, the Tibetans are above roughly 10% of the air molecules in the atmosphere.