The future size of the world population depends on how soon it reaches replacement-level fertility, the point at which each woman bears on average about 2.1 children. The graph shows projections for reaching that point in different years.

(a) Estimate the average rate of change in population for each projection from 1990 to 2050. Which projection shows the smallest rate of change in world population?
(b) Estimate the average rate of change in population from 2090 to 2130 for each projection. Interpret your answer.

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Population (billions) 12 11 10 a ∞0 6 5 Ultimate World Population Size Under Different Assumptions Replacement-level fertility reached by: 2050 2030 2010 + 1990 2010 2030 2050 2070 2090 2110 2130 Year