A machine with a cost of $180,000 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-of-output method. What is the amount of depreciation for the second full year, during which the machine was used 7,000 hours? A $7,000 B $63,000 C $77,000 D $45,000