Q. Assertion (A) Smaller the organism, higher is the rate of metabolism per gram weight.
Reason (R) The heart rate of six months old baby is much higher than that of an old person.

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Solution:

The metabolic rate per gram weight is higher in smaller organisms and consequently greater is the need for the pumping action of the heart (heart rate) to supply nutrients and oxygen to the tissues. The heart rate of a six month old body is much higher that of the old person because of high metabolic rate. Therefore both assertion and reason are true and reason is the correct explanation of the assestion.