A City Can Be Considered an Ecosystem as Shown in the Figure

A city can be considered an ecosystem as shown in the figure below. What is not shown in the figure that would make the city more sustainable?

A) The internal cycling of materials and energy within the urban system, which would reduce both the inputs necessary as well as the wastes produced.
B) A climate that supports outdoor activities year round, which would reduce the use of a car.
C) A population of 10 million residents or more, which would mean that less resources would be used per person.
D) High socio-economic levels and a population of similar ethnicity, which would mean that green decisions about city life could be made easily and people could afford to pay for them.