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

A．It is part of short-term memory.
B．It is useful in our remembering images.
C．It is important to our permanent storage of information.
D．It enables us to remember objects shortly.

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.