Mark and Charles are roommates at college. Each has written a 25-page term paper for the same English class. They are equally poor typists. Charles types his own paper and gets paid by Mark to type his, too. On the basis of the information given, which one of the following must be true?

a. Mark is wealthier than Charles.
b. Mark needs more time to study than Charles.
c. Charles hates typing.
d. Mark and Charles have different utilities for typing but place the same value on time.
e. Mark's opportunity cost of typing is higher than Charles's.

E