Reading Comprehension

For many years, theoretical economists characterized humans as rational beings relentlessly bent on maximizing purely selfish reward. Results of an experimental economics study appear to contradict this view, however. In the "Ultimatum Game," two subjects, who cannot exchange information, are placed in separate rooms. One is randomly chosen to propose how a sum of money, known to both, should be shared between them; only one offer, which must be accepted or rejected without negotiation, is allowed. If, in fact, people are selfish and rational, then the proposer should offer the smallest possible share, while the responder should accept any offer, no matter how small: after all, even one dollar is better than nothing. In numerous trials, however, two-thirds of the offers made were between 40 and 50 percent; only 4 percent were less than 20 percent. Among responders, more than half who were offered less than 20 percent rejected the offer. Behavior in the game did not appreciably depend on the players' GMAT, age, or education. Nor did the amount of money involved play significant role: for instance, in trials of the game that were conducted in Indonesia, the sum to be shared was as much as three times the subjects' average monthly income, and still responders refused offers that they deemed too small.

The primary purpose of the passage is to
A) provide evidence in support of the view that human beings are essentially rational and selfish
B) use a particular study to challenge the argument that the economic behavior of human beings may be motivated by factors other than selfishness
C) compare certain views about human nature held by theoretical economists with those held by experimental economists
D) describe a study that apparently challenges theoretical economists' understanding of human economic behavior
E) suggest that researchers may have failed to take into account the impact of certain noneconomic factors in designing a study of human economic behavior

Well, it seems like a simple question, doesn't it?

But for some reason, I'm having major issues with 'primary purpose' questions. Could you please provide me with a strategy to tackle such questions?

Will provide the OA after some discussion.