Please explain step by step how to get the answer to this question.

Question:

Suppose a firm uses labor and capital to produce output. The last unit of labor hired has a marginal

product of 12 units of output, and the last unit of capital employed has a marginal product of 20 units.

Use the optimal combination of inputs rule to calculate the price of capital if the price of labor is $6 per

unit. The price of capital is?

Select one:

a. $2

b. $10 - Correct

c. $20

d. $30