On april 1 of the current year, a company purchased and placed in service a machine with a cost of $240,000. the company estimated the machine's useful life to be four years or 60,000 units of output with an estimated salvage value of $60,000. during the current year, 12,000 units were produced. prepare the necessary december 31 adjusting journal entry to record depreciation for the current year assuming the company uses: a. the straight-line method of depreciation b. the units-of-production method of depreciation c. the double-declining balance method of depreciation