71. Which of the following best describes how a neuron fires? (A) Na+ ions cross the plasma membrane, initiating a wave that travels down the axon；(B) Vesicles carry neurotransmitters from the nucleus to the other end of the neuron；(C) Na+ ions enter one end of the neuron and diffuse to the other end, down the axon；(D) Neurotransmitters enter one end of the neuron and diffuse to the other end, down the axon.