In Paragraph 3, the author mainly employs______to illustrate his point.

A．motor vehicles are the main cause for air pollution in Mexico
B．there's a shortage of water supply and houses in Mexico
C．air pollution is serious in Mexico because of its location
D．urbanization leads to environmental problems in both developing and developed countries

A．More than half of African population will live in cities.
B．Many cities in Asia will have more than 20 million inhabitants.
C．Cities in the developing world will be even larger.
D．More cities in Asia have more than one million people.