Read the passage carefully, then choose the correct answers.
The dramatic growth of the world's population in the twentieth century was on a scale without parallel in human history. Most of this growth had occurred since 1950 and was known as the population 'explosion'. Between 1950 and 1980 the world population increased from 2.5 to over 4 billion, and by the end of the twenty century the figure had risen to about 6.6 billion. Growth of this size cannot continue indefinitely. Recent forecasts suggest that the total population will level out at between 10 and 15 billion in the mid twenty first century. Already there are encouraging signs that the rate of increase in many less developed countries is beginning to slow down.
In 2000, the world's population was about ______.