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ICSE Class 9 Geography Question 8 of 19

Composition and Structure of the Atmosphere — Question 11

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Question

Question 11

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.
Answer

Both A and R are true and R is the correct explanation of A.

Explanation — In atmosphere, there are concentric layers of air, each layer having different density. The upper layers press down the lower ones. So, the air of the lower layer is always heavier or denser due to which about 90 per cent of the mass of air lies within a height of about 20 km from the surface of the earth.