EM.67 Jo has already spent time preparing for an exam for 5 hours now and thinks they can earn a "C" on an upcoming exam. If they studied for 5 more hours, they can raise their grade but only to a "C+." This most closely relates to the economic concept of Select an answer and submit. For keyboard navigation, use the up/down arrow keys to select an answer. a allocation efficiency. b technical inefficiency. c increasing marginal costs. d diminishing marginal returns