Joe sold gold coins for $1,000 that he bought a year ago for $1,000. He says, "At least I didn't lose any money on my financial investment." His economist friend points out that in effect he did lose money because he could have received a 3% percent return on the $1,000 if he had bought a bank certificate of deposit instead of the coins. The economist's analysis in this case incorporates the idea of A) marginal benefits that exceed marginal costs. B) normative economics. C) imperfect information. OD) opportunity costs.