Why can depreciation expense be a different amount each year using the units-of-production method?

a. The method calculates expense based on the relationship between each year's output and total estimated output.

b. The method calculates expense based on cost divided by the expected useful life.

c. The units-of-production method is an accelerated depreciation method.

d. The units-of-production method is based on the difference in the appraisal value of the asset from one year to the next.