Read the passage, then choose the correct answers

The world's population increased from 3 billion in 1959 to 6 billion by 1999, a doubling that occurred over 40 years. The Census Bureau's lasted projections imply that population growth will continue into the 21^st century, although more slowly. The world's population is projected to grow from 6 billion in 1999 to 9 billion by 2042, an increase of 50 percent that will require 43 years.

The world's population growth rate rose from about 1.5 percent per year from 1950 to 1951 to a peak of over 2 percent in the early 1960s due to reductions in mortality. Growth rates thereafter started to decline due to rising age at marriage as well as increasing availability and use of effective contraceptive methods. Note that changes in population growth have not always been steady. A dip in the growth rate from 1959 to 1960, for instance, was due to the Great Leap Forward in China. During that time, both natural disasters and decreased agricutural output in the wake of massive social reorganization caused China's death rate to rise sharply and its fertility rate to fall by almost half.

How many people did the world's population increase between 1959 and 1999?

• A. 
  3 billion
• B. 
  6 billion
• C. 
  40 billion
• D. 
  9 billion