The authors mentions all the following as causes of declines in population growth after 19

The authors mentions all the following as causes of declines in population growth after 1957 EXCEPT

A．people"s staying at school longer.

B．people"s getting married earlier.

C．better standards of living.

D．couples" buying houses before starting families.

请帮忙给出正确答案和分析，谢谢！