A Factory Machine Was Purchased for $90,000 on January 1

A factory machine was purchased for $90,000 on January 1, 2011.It was estimated that it would have a $18,000 residual value at the end of its 5-year useful life.It was also estimated that the machine would be run 40,000 hours in the 5 years.The company ran the machine for 4,000 actual hours in 2011.If the company uses the units-of-activity method of depreciation, the amount of depreciation expense for 2011 would be

A) $9,000.
B) $14,400.
C) $18,000.
D) $7,200.