Doctor Hugh R. Sick recommends his patient take 2 units of insulin per day to successfully manage her diabetes condition. In a different office across town, Doctor I. M

Dismal, an economics professor, tells his student to read every assigned textbook chapter to successfully pass economics. In spite of the sage advice of the doctor and professor, the economic way of thinking recognizes A) patients don't always follow their doctor's orders.
B) students don't always follow their professor's pleas.
C) substitutes for prescriptions and textbook assignments exist; patients and students demonstrate their desire for substitutes through their actions.
D) all of the above are true.

D