Where in the passage does the author compare the energy consumption of skyscrapers with that of a city

A．compare skyscrapers with other modern structures
B．describe skyscrapers and their effect on the environment
C．advocate the use of masonry in the construction of skyscrapers
D．illustrate some architectural designs of skyscrapers

A．The exterior surrounding air is heated.
B．The windows must be cleaned daily.
C．Construction time is increased.
D．Extra air-conditioning equipment is needed.