Windsor Company purchased a depreciable asset for $300,000 on April 1, 2005. The estimated salvage value is $30,000, and the asset is expected to produce 135,000 units of product in its estimated useful life. The asset actually produces 18,000 units, 12,000 units and 7,000 units in years 2005, 2006 and 2007, respectively. The units-of-production method is used for depreciation. How much depreciation expense should be recognized for 2006?

a. $36,000 b. $22,800 c. $14,000 d. $24,000

I know the is answere is D. Please explain. Thank you!