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The graph below shows the marginal benefit per hour of studying economics (points earned on exam) versus the marginal cost (points lost on other course exams because of less time spent studying for other courses). If you study economics for 6 hours, the marginal benefit is points, and the marginal cost is points. Additional points on an exam 40 MC 30 20 10 MB 0 2 4 6 8 Hours studying economics Select one: a. 20; 0 O b. 20; 30 O c. 30; 10 O d. 10; 30