@Parker-Zopp the problem with (A) is that a decrease in the average number of children per family could be explained by an increase in the number of families having zero children.
For example, imagine that the average number of children dropped from 4 to 2 in the following way:
Before: 4 4 4
After: 4 4 4 0 0 0
Notice in this example, the number of children in large families has remained the same.
The key here is comparing children in large families with children in small families.
Hope that helps! Please let us know if you have any other questions.
