Equipment with a cost of $160,000, an estimated residual value of $40,000, and an estimated life of 15 years was depreciated by the straight-line method for 4 years. Due to obsolescence, it was determined that the remaining useful life should be shortened by 3 years and the residual value changed to zero. The depreciation expense for the current and future years is Oa. $11,000 Ob. $16,000 Oc. $8,000 Od. $11,636