Why did high fertility of the early population occur, according to Hug

Why did high fertility of the early population occur, according to Hughes and Cain (2011)?

(a) More than half of the population were at ages where fertility is high—in 1820, for example,
the median American was less than 17 years old.
(b) A nation composed mainly of farmers and people planning to farm had every reason to create large families.
(c) Children can be considered investment goods and the rate of return on a child born to a farm family was relatively high.
(d) High fertility occurred due to all of the above reasons.

ANSWER

(d)