In this layer of earth's atmosphere, hydrogen and helium are the prime gases and are only present at extremely low densities. what is this layer of the atmosphere called? a) exosphere b) ionosphere. c) mesosphere d) thermosphere

The answer would be the EXOSPHERE. The exosphere is the highest layer of the atmosphere. This layer merges with space and the air is extremely thin. Molecules of air here always escape from the from the exosphere to space. This is also the layer where satellites orbit the Earth. 

The answer a. 

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