It Took More Than 100 Years for the World's Population

It took more than 100 years for the world's population to double from 1 billion to 2 billion. How long did it take for the world's population to increase from 4 billion to 5 billion?

A) Fewer than 15 years as the result of exponential growth
B) Half the time of the first doubling, about 50 years
C) More than 100 years; 1 billion people are added each 100 years
D) The world's population has not yet reached 5 billion