The correct option is B It grew steadily from 1951 to 1981 and gradually decreased from 1981 to 2011.
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Between 1951 to 1981, the annual rate of population growth was steadily increasing - the population grew from 361 million in 1951 to 683.3 million in 1981. This growth took a gradual downward turn from 1981 to 2011. Though the population growth rate has seen a downward trend, the absolute population is still increasing.