Choice "D" is correct. Sum-of-the-years-digits: 3 + 2 + 1. Third year/6 x $9,000$1,500.
Choice "c" is incorrect. Under the straight-line method, the calculations would be: $12,000 - $3,000$9,000; $9,000 ÷ 3$3,000. Thus, this is not the correct answer.
Choice "b" is incorrect.
Choice "a" is incorrect. There is not enough information to calculate depreciation using the units-of-production method.Note: The best way to determine this answer is by process of elimination. Compute the depreciation for the simplest methods first, and stop as soon as the correct answer is identified. Under this elimination approach, it would not have been necessary to compute depreciation using the double-declining balance method.