Bio30 questions please answer :(

The nerve impulses always travel in one direction from dendrites to axon terminals. What prevents an action potential from travelling backward?
Select one:
a. After repolarization, the refractory period occurs, where the sodium ions are found outside the cell.
b. The refractory period prevents another action potential by lowering the threhold potential.
c. After repolarization, the reversal of ions prevents another action potential as there would not be enough sodium ions outside the cell to rush in.
d. Hyperpolarization of the neuron makes the membrane more positive and prevents another action potential.

Which of the following situations correctly illustrates the All or None Principle?
Select one:
a. A smaller voltage change will result in a smaller nerve impulse.
b. Greater voltage change and smaller voltage change will result in the same action potential as long as the threshold potential has been reached.
c. A faster impulse can be generated with a voltage change that is greater than the threshold potential.
d. A stronger impulse can be generated by increasing the number of open voltaged gated sodium channels during depolarization.