Unidirectional transmission of a nerve impulse through nerve fibre is due to the fact that
(a) nerve fibre is insulated by a medullary sheath
(b) sodium pump starts operating only at the cyton and then continues into the nerve fibre
(c) neurotransmitters are relesed by dendrites and not by axon endings
(d) neurotransmitters are released by the axon endings and not by dendrities.