In 1900, the world had a human population of approximately _______________; in 2000, the world had a human population of approximately _______________.
a. 4.3 billion; 6.7 billion
b. 550 million; 7.4 billion
c. 2.8 billion; 4.4 billion
d. 1.6 billion; 5.2 billion
e. 1.6 billion; 6.1 billion