The Zorro Company Purchased Equipment on January 1, 2010, for $100

The Zorro Company purchased equipment on January 1, 2010, for $100, 000.The equipment had an estimated residual value of $10, 000, an estimated useful life of five years, and estimated lifetime output of 18, 000 units.In 2011, the company produced 4, 400 units and recorded depreciation expense of $22, 000.What depreciation method did the company use?

A) straight-line method
B) sum-of-the-years'-digits method
C) double-declining-balance method
D) units-of-output method