On January 1, 20X5, Zipper Manufacturing Company purchased a machine for $39,980, and expects to use the machine for a total of 32,000 hours over the next four years. Zipper set the residual value on the machine at $3,500. Zipper used the machine 6,000 hours in 20X5 and 7,200 hours in 20X6. What is the depreciation expense for 20X6 if Zipper uses double-declining balance depreciation?

a. $9,995 b. $6,000 c. $6,875 d. $12,000