Assertion (A): Photochemical smog is formed in warm, dry and sunny weather. Reason (R): Hydrocarbons are present in vehicle exhaust.

ABoth A and R are true and R is the correct explanation of A
BBoth A and R are true but R is not the correct explanation of A
CA is true but R is false
DA is false but R is true

Answer: B

Photochemical smog needs sunlight to photolyse

NO X_{2}

, so warm sunny weather favours it; A is true. Exhaust does contain hydrocarbons, so R is true. But the presence of hydrocarbons does not explain why sunny, dry weather is needed (that is the photolysis), so R is not the explanation of A.

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