The United Nations Periodically Publishes Population Projections for the 21^st

The United Nations periodically publishes population projections for the 21^st century. In their most current projection (shown in the graph below) , their estimates range from a high estimate ( 10.2 billion in 2050) to a low estimate (9.3 billion in 2050) . What is the reason for this range of population numbers?

A) The estimates project what we hope will happen (low estimate) versus what we expect will happen (high estimate) .
B) The estimates vary depending on fertility changes, particularly in less developed countries, because that is where almost all of the growth will take place.
C) The estimates are based on how epidemics such as AIDS, SARS, and swine flu progresses affecting future death rates.
D) The estimates vary depending on the population model used.