Which of the following best describes the relationship between diminishing marginal returns and marginal cost? a. if marginal returns are diminishing while output increases, marginal cost must be increasing. b. if marginal returns are diminishing while output increases, marginal cost must be decreasing. c. if marginal returns are diminishing while output increases, marginal cost can be either increasing or decreasing. d. marginal returns diminish as output decreases, and thus marginal cost must increase. e. marginal returns diminish as output increases, and thus marginal cost must decrease.