Amy is training for a competition, a timed race that combines swimming, rowing, and running. Consider the following statement: Because her pool sessions are helping her swim more quickly, Amy plans to reduce by $30$ minutes per week the time she spends training on rowing and increase by $30$ minutes the time she spends in the swimming pool; however, her bff says that she should stop doing any rowing and running and spend all $15$ hours per week training in the pool.

Which concept does Amy's plan demonstrate that her bff$\text{'}$s advice does not?

People generally take advantage of opportunities for self$-$gain

Many decisions are made on the margin.

Markets are usually efficient

Everything has a cost