1. Jack sold a baseball card collection for $2,500 that he bought a year ago for $2,500. He says, "At least I didn't lose any money on my investment in this baseball card." His economist friend points out that in effect he did lose money because he could have received a 3 percent return on the $2,500, which is $75, if he had deposited the money in a bank. The economist's analysis in this case incorporates the idea of: