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.