In some cultures, children are encouraged to walk earlier or later than what would be considered the ...

In some cultures, children are encouraged to walk earlier or later than what would be considered the average age in the United States (Super, 1976). With this in mind, which of the following statements is accurate?
 
  a. Changes in the developmental timeline are virtually imperceptible because motor milestones are genetic.
  b. Changes in the developmental timeline are generally quite dramatic depending on the culture.
  c. Changes in the developmental timeline are usually slight because myelination of motor nerves usually just precedes the achievement of a new skill.
  d. Changes in the developmental timeline can take place only during the spurt of myelination that occurs between ages 6 and 13.