Examination of the Ages at Which Children Achieve Motor Milestones

Examination of the ages at which children achieve motor milestones shows that:

A) practice has little effect on the ages at which babies master universal skills.
B) 90 percent of infants have begun walking by the time they are 12 months of age.
C) there are wide variations in the ages at which infants begin to sit, walk, and crawl.
D) infants are remarkably uniform in the ages at which they become able to sit, crawl, and walk.