All of the following are examples of opportunity cost except:
A. the leisure time sacrificed to study for an exam.
B. the tuition fees paid to a university.
C. the income which could have been earned by a college student had he or she worked full time instead of attending college.
D. the building which could have been built with the construction materials and labor used to build a new university library.
E. all of the above are examples of opportunity costs.