Mothers with Young Children Work Fewer Hours Than Mothers with Older

Mothers with young children work fewer hours than mothers with older children. Which of the following could explain this?

A) Young children cost more in real goods and services.
B) Mothers with young children have husbands that make less.
C) Mothers with young children have flatter utility isoquants (where household time is on the horizontal axis) .
D) Mothers with young children have steeper utility isoquants (where household time is on the horizontal axis) .