The Clarke Co. acquired a machine on May 1, 2006, at a cost of $60,000. The machine is expected to have a ten-year life and a residual value of $5,000. The estimated lifetime output from the machine is expected to be 55,000 units. Under which of the following depreciation methods would the depreciation charge be the greatest in 2006, if 9,100 units were produced in that year?