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. opportunity costs
b. marginal benefits that exceed marginal costs
c. imperfect information
d. normative economics