Equipment with a cost of $52,920, an estimated residual value of $2,520, and an estimated life of 14 years was depreciated by the straight-line method for 6 years. Due to obsolescence, it was determined that the remaining useful life should be shortened by 5 years and the residual value changed to zero. The depreciation expense for the current and future years is Oa. $10.440 Ob. $2.237 Oc. $2.847 Od. $21.600