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18. Suppose Sean always maximizes his utility. Last week, Sean ate 6 hamburgers and drank 3 milkshakes. This week, when the price of hamburgers increased, Sean ate 5 hamburgers and drank 2 milkshakes. Sean's willingness to give up milkshakes to get hamburgers: A. is higher this week than last week. B. is the same both this week and last week. C. could be either higher or lower this week compared to last week. D. is lower this week than last week.