Candice is planning her activities for a hot summer day. She would like to go to the local swimming pool and see the latest blockbuster movie, but because she can only get tickets to the movie for the same time that the pool is open she can only choose one activity. This illustrates the basic principle that

a$.$ people face tradeoffs.

b$.$ improvements in efficiency sometimes come at the expense of equality.

c$.$ people respond to incentives.

d$.$ rational people think at the margin.