A building with a cost of $500 000, an estimated residual value of $50 000 and an estimated useful life of 20 years was depreciated by the straight-line method for 10 years. In the 11th year, it was determined that the useful life should be extended by 10 years (that is, from 20 years to 30 years). The residual value was to remain the same. The depreciation expense for the current and future years is: Select one: O a. $12 500 O b. $13750 O c. $11 250 O d. $15 000