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