What, in addition to geographic boundaries, might have kept the people of the Huang River civilization from traveling to far-distant regions?


a
They feared that their bronze weapons would be no match for the iron weapons of ancient Egypt.

b
They could get everything locally that they needed to survive and grow.

c
They had learned about the other river valley civilizations but feared contact would bring invasion.

d
They had not yet developed a system of money based on coins.