Which of the statements about the law of diminishing marginal returns is correct?

a.As the size of a firm's plant increases, average cost eventually decreases.

b.As a firm uses more of a variable input, holding the quantity of fixed inputs constant, the marginal product of the variable input eventually diminishes.

c.As a firm makes more investment in capital, marginal product of capital eventually decreases.

d.As a firm hires more workers, total product eventually decreases.