Explain why the gates in the Great Wall of China became center of trade?

The Mongols were nomadic people who lived north of China. Mongol raids into China caused a great deal of destruction during the Age of Warring States. ... Gates through the wall became centers of trade and contact with the northern nomads

Explanation:

im pretty sure the answer is b, i hope i < 3

2018 means 2018 ce or common era

there have been other eras too including bce, which included way more years.