Afactory machine was purchased for $387000 on January 1, 2021. It was estimated that it would have a $72000 salvage value at the end of its 5-year useful life. It was also estimated that the machine would be run 36000 hours in the 5 years. The company ran the machine for 3600 actual hours in 2021. If the company uses the units-of-activity method of depreciation, the amount of depreciation expense for 2021 would be O $31500. O $63000. O $38700. O $72000