It took（）years for world population to grow from 1 billion to 3 billion.

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．If man can live longer in the future
B．If the science of medicine is developing fast enough
C．If the population growth is not reduced
D．If the earth is able to provide a good life for more people