A company purchased a delivery truck on January 1, 2013, for $90,000. It is estimated that the delivery truck will have a $16,000 salvage value at the end of its 4-year useful life. If the company recorded depreciation expense of $22,500 for the year 2014 on the delivery truck, the depreciation method used by the company is:

a. the straight-line method.

b. the double-declining-balance method.

c. the units-of-activity method.

d. not determinable.