Kipsigis and Jamaican Infants Walk Considerably Earlier Than North American

Kipsigis and Jamaican infants walk considerably earlier than North American infants because

A) they are physically smaller at birth, requiring less leg-muscle strength for standing.
B) they must help their parents work in the fields at an extremely young age.
C) they are genetically "hardwired" to do so.
D) their parents promote early walking instruction, practicing formal exercises to stimulate skills.