Assertion (A): About 90 per cent of the mass of air lies within a height of about 20 km from the surface of the earth.

Reason (R): The air of the lower layer is always heavier or denser, while the upper layers are thinner or less dense.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R does not explain A.
3. A is true but R is false.
4. A is false but R is true.

Name the uppermost layer in the structure of the atmosphere that gradually merges with interplanetary space.

1. Ionosphere
2. Exosphere
3. Troposphere
4. Mesosphere

Clouds and weather phenomenon occur in the Troposphere but some clouds are found in the Mesosphere. Why?

1. Meteoric dust particles reflect sunlight causing wispy clouds.
2. Some water particles reach the Mesosphere to form clouds.
3. Ions from the ionosphere deflect sunlight giving the impression of clouds.
4. None of the above.

Which of the following are the functions of the atmosphere?

1. Facilitates the cyclic exchange of gases
2. Maintain of optimum temperature
3. Protects the earth from harmful solar radiation
4. All of the above.