Marcia finds that the net benefit of flying from Chicago to Honolulu on a non$-$stop United Airlines flight is $$400,$ and the net benefit for the same trip flying on a one$-$stop American Airlines flight is $$200.$

A$.$ Optimization in differences, since she is calculating the change in net benefits between alternatives.

B$.$ Optimization in levels, since she is comparing the net benefits between alternatives.

C$.$ Neither type of optimization, since she is considering the net benefits of alternatives.

D$.$ Optimization in differences, since she is making marginal comparisons.

E$.$ Both types of optimization, since she is not considering the total benefits of alternatives.