Problem:
If five years ago, a company purchased a machine for $30,000. The machine is being depreciated over a ten-year useful life, using straight-line depreciation. Cumulative depreciation is $15,000. The current net book value is $15,000 and the current market value is $18,000. The original purchase price of $30,000 be:
a. a period cost
b. a opportunity cost
c. a variable cost
d. a sunk cost