Equipment with a cost of $71,394, an estimated residual value of $4,312, and an estimated life of 13 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 4 years and the residual value changed to zero. The depreciation expense for the current and future years is Oa. $4,043.31 Ob. $3.110.24 Oc. $13,477.69 Od. $30.960.92