Question

Please help! Will Fan and Medal!
The Nan Song was the first dynasty to allocate a standing army at a port of trade for protection. What would be the reason for doing this?

to protect all concerned with the trade industry thus insuring its continuation
to protect the citizenry from foreign traders who were known to be brutal and barbarous
to protect the borders of China from illegal entry of foreigners
to protect foreign traders from a citizenry who did not like any foreign influence on their country