Suppose that the extra cost to Tim of a third glass of soda is zero because he’s at a restaurant that gives free refills. According to the Cost-Benefit Principle Tim should:

A.not drink a third glass of soda.

B.drink a third glass of soda if his total benefit from drinking soda is positive.

C.drink a third glass of soda.

D.drink a third glass of soda if the extra benefit of doing so is positive.

正确答案：drink a third glass of soda if the extra benefit of doing so is positive.