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After running for a while John observes that his heart is beating very fast. He made the following hypothesis for finding the reason of fast heart beating. Choose which one of the following hypothesis is correct?

A)  Heart is actually suspended into thoracic cavity. Therefore, on running, heart starts vibrating back and forth. And it feels like fast heart beating.

B)  Legs are the organ responsible for running. Legs are below the heart. Therefore, heart do not require any force to send the blood in the legs. John might have some heart problem, that is why his heart was beating fast.

C)  Muscles, responsible for running, worked hard while running. Therefore they required great amount of energy. Cells required faster transportation of required materials. Therefore, hearts started to beat fast in order to fast the circulation of blood.

D)  Actually it is nothing overall. He got tired of running therefore he feels so.

E)  None of these