What Is the Meaning of "Units-Of-Production Method

What is the meaning of "units-of-production method"?

A) The systematic allocation of an asset's depreciable amount allocated in proportion to the productive capacity used.
B) The systematic allocation of an asset's depreciable amount allocated evenly over the asset's estimated useful life.
C) The systematic allocation of an asset's depreciable amount over its estimated useful life.
D) The systematic allocation of an asset's depreciable amount whereby a period's depreciation equals the asset's net carrying amount multiplied by a fixed percentage.