13. Assume a firm's inputs to production are labor and production facilities. Diminishing marginal returns does not apply to labor when all inputs to production are variable because a . eventually marginal product of labor will always begin to increase again, regardless of the type of inputs. b. a firm can build an additional production facility so each worker's share of the facility doesn't necessarily decrease. c. a firm can hire inefficient workers. d. none of the above, diminishing returns applies when all inputs to production are variable