The World's Population Is Two and a Half Times Larger

The world's population is two and a half times larger than it was in 1950.This is because:

A) the fertility rate has been increasingly dramatically.
B) the birth rate and the fertility rate have both increased, making population growth double.
C) mortality as decreased rapidly while fertility has decreased only slowly.
D) None of these; migration is the only factor that has changed since 1950.