A Firm Is Producing 10,000 Units of Output in Two

A firm is producing 10,000 units of output in two plants,A and B,and each plant is producing 5,000 units of output.The marginal cost in plant A is $10 and the marginal cost in B is $6.To reduce the cost of producing 10,000 units the firm should

A) produce more in A and less in B.
B) produce less in A and more in B.
C) produce it all in B because B is the lower cost plant.
D) do nothing since the output in each plant is equal.