Question: Jill is training for a triathlon, a timed race that combines swimming, biking, and running.

Consider the following statement: "Because her pool sessions are helping her swim much faster, Jill plans to reduce by one hour per week the time she spends training on the bike and increase by one hour the time she spends in the swimming pool; however, her husband says that she should stop doing any biking and running and spend all 20 hours per week in the pool."

Which basic principle of individual choice does Becky's decision illustrate that her husband's advice does not?
A)Resources are scarce.
B)Many decisions are taken at the margin.
C)All costs are opportunity costs.
D)People usually exploit opportunities to make themselves better off.  Edit

Answer: B)Many decisions are taken at the margin.   Edit