The graph shows the world human population from 10,000 BCE to 2000 CE.

Graph of world human populations (e s t) from 10000 B C E to 2000 C E in billions. Population stayed steady in 10000, 8000, 6000, and 4000 B C E. Population started to rise in 2000 B C E and 1 CE. Population rose dramatically after.

Based on the graph, what was the estimated population size in 2000?

one billion
three billion
five billion
six billion