Answer
b. 1999
Work Step by Step
For the human population to reach 1 billion, it took 100,000 years. Because of a decline in mortality rates with no decline in birth rates around the world, the rate of growth has been steadily increasing. To get the human population to increase from 5 billion to 6 billion people, it only took about 12 years. The world population reached 6 billion people in 1999.
