Question

 At the beginning of which century the world population was one billion? ​

Answer 1

Answer:

in 19th century

Explanation:

It is estimated that the world population reached one billion for the first time in 1804. It was another 123 years before it reached two billion in 1927, but it took only 33 years to reach three billion in 1960.

Answer taken from Google

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Answer 2

Answer:

At the beginning of the century china has the world population of one billion.