The Bay Area in California Is Home to the Oakland

The Bay Area in California is home to the Oakland Athletics and the San Francisco Giants. New York City is home to the Mets and Yankees. Most other cities only have one baseball team, if any at all. What is the best explanation for this phenomenon?

A) Most baseball fans, who are the demanders for baseball games, tend to live in New York and San Francisco.
B) Baseball team owners live in large cities, such as New York and San Francisco.
C) In most baseball cities-excluding New York, San Francisco, etc.-baseball teams are natural monopolies.
D) Only these four teams face downward-sloping demand curves.
E) Only these four teams have declining average total costs.