What was the significance of the decline in fertility rates in America

What was the significance of the decline in fertility rates in America between 1800 and 1939?

a. It reflected a higher death rate among immigrant families.
b. It reflected a higher infant mortality rate due to primitive medical practices.
c. It reflected a conscious decision of many Americans to postpone or limit their families.
d. It showed that more Americans were remaining single rather than marrying and having children.
e. It showed that fewer Americans were dying of disease now that vaccines had come into popular use.

ANSWER
C