According to the article, the factor that contributing to the fall in birth rates today is D. People in rural areas are moving to new areas for better job opportunities.
The birth rate in a period simply refers to the total number of newborns that are given birth to in a geographical area.
It should be noted that naturally, women in the rural areas give birth frequently more than those in the cities since they're less exposed. Therefore, since the people in rural areas are moving to new areas, this has led to a reduction in the birth rates.
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