A machine with a cost of $240,000 has an estimated salvage value of $20,000 and an estimated useful life of 5 years or 16,000 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 4,500 hours? a. $45,000 b. $48,000 c. $61,875. d. $96,000