For which axon might you expect action potential propagation to be the fastest?

For which axon might you expect action potential propagation to be the fastest?
 
  a. a myelinated axon with a small diameter
  b. an unmyelinated axon with a small diameter
  c. a myelinated axon with a large diameter
  d. an unmyelinated axon with a large diameter
  e. axon myelination and diameter do not affect action potential propagation rate

Question 2

A reasonable estimation of the duration of an action potential is
 
  a. approximately five microseconds.
  b. approximately five milliseconds.
  c. approximately five centiseconds.
  d. approximately five seconds.
  e. approximately five nanoseconds.