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D is corrent because ASC Topic 350 states that periodic review of amortization policies should be undertaken and required changes should be made prospectively. When the asset was acquired on January 1, year 1, it was to be amortized over its useful life. The asset was determined to have a remaining useful life of 30 years at January 1, year 6. The unamortized cost at that date should be amortized over the remaining useful life of 30 years.
A is incorrect because it implies a retroactive restatement of amortization, and ASC Topic 250 requires that a change in estimate be treated prospectively.
B is incorrect because revisions in useful life should result in revisions of amortization rates.
C is incorrect because the useful life at 1/1/Y6 is 30 years, and to use a life of 40 years would violate conservatism and overstate income.