A machine with a cost of $80,000 has an estimated salvage value of $5,000 and an estimated useful life of five years or 15,000 hours. It is to be depreciated using the units of activity method of depreciation. What is the amount of depreciation for the second full year, during which the machine was used 5000 hours?

a. $25,000 b. $15,000 c. $21,667 d. $26,667