Afundamental source of monopoly market power arises from

a. barriers to entry
b. perfectly elastic demand
c. availability of "free" natural resources, such as water or air
d. perfectly inelastic demand

Barriers to entry

Explanation:

Monopolies arise when the barriers to entry for competitors is simply too high. Think about power companies. It would be almost impossible for a secondary power company to offer service due to the infrastructure costs alone. If companies can make it difficult for competitors to enter a market, monopolies occur.

they can see what that city is producing and to also see if they can some how get that product. like gold in africa.