A machine with a cost of $144,000 has an estimated residual value of $18,000 and an estimated life of 4 years or 18,000 hours. It is to be depreciated by the units-ofactivity method. What is the amount of depreciation for the second full year, during which the machine was used 5,200 hours? a. $5,200 b. $18,400 c. $36,400 d. $46,800