The following table shows the total revenue and total cost (in dollars) from different sales volumes of the goods.

Table 2-1

15 1 15 3

14 2 28 7

13 3 39 12

12 4 48 18

11 5 55 25

10 6 60 33

Refer to Table 2-1. What is the marginal profit of the firm from the sale of the 3rd unit of the good?

a. $9

b. $6

c. $4

d. $2

e. 0