The primary purpose can be determined only by evaluating the whole passage. The first paragraph discusses consumers' perceptions of quality based on frequency of advertising. The second paragraph discusses three studies that show how consumers base their evaluations of products on the kinds of advertising they see. Therefore, the purpose of the whole passage is to show how consumers' perceptions of products are shaped by certain advertising practices.
A The passage shows that expensive advertising works to a certain point, but not after it; this method examines a continuum, not a contradiction.
B Most of the passage is devoted to empirical evidence.
C Correct. The passage shows how the frequency and the kind of advertising influence consumers' perceptions about the quality of the products advertised.
D The passage reports the findings of four studies but does not mention research methods.
E The passage does not indicate that any of the findings were unexpected. The correct answer is C.