Question: There are two billion children in the world today (2020), aged 0 to 15 years old. How many children will there be in the year 2100 according to the United Nations?
A: 4 billion
B: 3 billion
C: 2 billion
D: 1 billion
Please eliminate two incorrect options first, then think it step by step and choose the most proper one option. Provide the entire answer in JSON format.
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