Final answer:
It took approximately 123 years for the human population to go from 1 billion to 2 billion, consistent with exponential growth.
Step-by-step explanation:
The human population took different amounts of time to reach each billion milestone. From 1 billion to 2 billion, it took approximately 123 years. This growth pattern is consistent with exponential growth as each billion has taken less time to reach compared to the previous billion. For example, it took 24 years to add 2 billion people between 1975 and 1999.