If the Price of a Textbook Rises and Then Students

Multiple Choice

If the price of a textbook rises and then students purchase fewer textbooks, an economic model can show a cause-and-effect relationship only if which of the following conditions holds:

A) students' incomes fall.
B) tuition decreases.
C) the number of students increases.
D) everything else is constant.
E) the bookstore no longer accepts used book trade-ins.