At Seven Months, Xiang Could Sit Up by Herself, at 9

At seven months, Xiang could sit up by herself, at 9 months she could stand with an adult's support, at 13 months she could stand alone, and at 15 months she could walk alone. The process that allows Xiang's motor skills to develop at the same rate as other children's is called

A) the proximodistal principle.
B) the cephalocaudal principle.
C) maturation.
D) accommodation.