Which of the following statements regarding supply and production function are correct? (A) In the short run, a firm can only adjust variable inputs, while in the long run, all inputs can be adjusted. (B) The law of diminishing returns applies when additional units of a variable input are added to a fixed input, resulting in a decline in marginal product. (C) The production function shows the maximum output a firm can produce with a given set of inputs. (D) Returns to scale refer to the relationship between changes in input quantities and resulting changes in output in the long run.