By the year 2000, today's ______ nations will have a total population of 5 billion people, nearly four fifth of the world's population.

A．a lot of social problems
B．medical advances
C．a decrease in food production
D．a worse life in every nation with 3 billion

A．the United Nations thinks it possible to avoid a population explosion
B．5.1 billion people are living in the underdeveloped countries
C．in advanced nations the population growth is being controlled
D．the world's population is increasing.