Why Did the Populations of Richer Nations Grow More Slowly

Why did the populations of richer nations grow more slowly than the populations of poorer nations at the end of the twentieth century?

A) Access to birth control, education, and better career prospects encouraged women to postpone childbearing.
B) The expense of raising children became so prohibitive that few people wanted to undertake it.
C) Effective border controls ended most international migration, eliminating a factor that had inflated population growth.
D) Cities were unhealthy places to live, so many children died at young ages.