Which of the following statements best explains why cities are so densely populated?

a.

cities have good airports for traveling.

b.

farms are the most densely populated areas of the country.

c.

cities have fewer government officials and less police protection than other regions.

d.

cities offer a wealth of opportunities for citizens, including jobs, access, education, and healthcare.

The best and most correct answer among the choices provided by the question is the fourth choice "Cities offer a wealth of opportunities for citizens, including jobs, access, education, and healthcare."
Population density (in agriculture : standing stock and standing crop) is a measurement of population per unit area or unit volume; it is a quantity of type number density. It is frequently applied to living organisms, and particularly to humans. It is a key geographic term.

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high entry costs prevent new producers from entering the market.

explanation:

an oligopoly is a term used to clarify the structure of a particular market, industry, or organization. a market is considered oligopolistic or amazingly focused when it is shared between a few companies.