The world's population is expected to increase by two billion people, from 7.8 billion at present to 9.8 billion in 2050, before reaching a peak of nearly 11 billion by the end of the century as fertility rates continue to decline. During this period, the global population is projected to become more and more urban, while children below age 5 will be outnumbered by persons aged 65 or above. According to the above information the current world population is approximately 7.8 billion. List the approximate years the world population reached one billion, two billion, three billion, four billion, five billion, six billion, and seven billion. Is this an example of exponential growth or linear growth? Explain your answer.