Machinery acquired new on January 1 at a cost of $200,000 was estimated to have a useful life of 10 years and a residual salvage value of $20,000. Straight-line depreciation was used. On January 1, following six full years of use of the machinery, management decided that the estimated useful life had been too long and that the machinery would have to be retired after three more years, that is at the end of the ninth year of service. Additionally, the market for the equipment had changed and management estimated that the salvage value at the end of the ninth year would be $44,000. Under this revised estimate the depreciation expense for the seventh year of use would be: Select one: a. $18,000 O b. 16,000 O c. $20,000 O d. $15,600