In the context of population growth, the doubling time of human population shortened from 200 years (to reach 2 billion) to approximately how many years to add the next billion after 5 billion?

Correct answer: C. 13 years

Explanation: The human population reached 5 billion in 1987 and 6 billion in 2000 - a doubling-time compression showing that adding the next billion took only about 13 years, demonstrating accelerating population growth.