A machine with a cost of $180,000 has an estimated residual value of $20,000 and an estimated life of 4 years or 20,000 hours. It is to be depreciated by the units-of- activity method. What is the amount of depreciation for the second full year, during which the machine was used 5,500 hours? Oa. $24,000 Ob. $44,000 Oc. $5,500 Od. $55,000