During the first period of demographic history, societies were often in danger of extinction because_________.

A．There might be an increase of 2.2 billion persons from now to the year 2000.
B．About 50,000 babies are born every, six hours at present.
C．Between 8000 B.C. and the present, the population increase is about 80,000,000 persons each year.
D．The population increased faster between 8000 B. C. and 1650 than between 1650 and the present.

A．warn people against the population explosion in the near future
B．compare the demographic growth pattern in the past with that after 1650
C．find out the cause for rapid increase in population in recent years
D．present us a clear and complete picture of the demographic growth