The Nomadic Horsemen of Central Asia Had What Crucial Advantage

The nomadic horsemen of Central Asia had what crucial advantage when acting as trade intermediaries between far-flung urban centers?

A) They had unique knowledge of the mountain passes through which trade goods flowed.
B) They developed political ties with dozens of local kings who allowed them to carry trade goods.
C) They had acquired resistance to many diseases because of close contact with their animals and interaction with many peoples.
D) They had learned many languages through their long history of interactions with settled peoples.