What Makes the Impulse Faster

What makes the impulse faster?

A) The larger the diameter of the axon,and the more myelin surrounding its outer surface,the faster the impulse.
B) Scientists have not been able find out till date what is the exact reason that makes an impulse faster.
C) The smaller the diameter of the axon,and the more myelin surrounding its outer surface,the faster the impulse.
D) The larger the diameter of the axon,and lesser the myelin surrounding its outer surface,the faster the impulse.