Which has the highest speed of nerve impulse?

What does the axoplasm of an axon contain in a resting state?

When a nerve fibre is stimulated,the inside of the membrane becomes:

$A$ nerve impulse will travel through a nerve fibre only if the membrane suddenly becomes more permeable to

The potential difference in the membrane,which is responsible for the conduction of an impulse,is brought about by a change in the membrane:

Which of the following statements is/are correct?
$(a)$ The electrical potential difference across the resting plasma membrane is called the resting potential.
$(b)$ $Na^+$ ions are responsible for the generation of an action potential.
$(c)$ $A$ nerve impulse is an action potential.