15. The table below shows the relationship between the amount of labor employed ($L$) and the average product.

| 勞動投入數量 | 1 | 2 | 3 | 4 | 5 |
| --- | --- | --- | --- | --- | --- |
| 平均產量 | 8 | 7 | 6 | 5 | 3 |

Which of the following is the marginal product of labor when $L = 3$?
(A) -1
(B) 2
(C) 4
(D) 6

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