A factory machine was purchased for $20,000 on March 1, 2021. It was estimated that it would have a $ 4,000 residual value at the end of its 5-year useful life. It was also estimated that the machine would be run 25,000 hours in the 5 years. If the actual number of machine hours run in 2021 was 4,000 hours and the company uses the units-of-production method of depreciation, the amount of depreciation expense for 2021 would be O a $2,133 O b. $2,560 Oc. $3,200 O d. $4,000