Suppose a country is able to produce a maximum of either 300 units of lumber or 100 units of rice. This country is currently allocating its labor resources to produce 75 units of lumber and 75 units of rice. To increase its lumber production by 6 units to 81, the country faces an opportunity cost of

A. 2 units of rice.

B. 6 units of rice.

C. 75 units of rice.

D. 18 units of rice