When computing DD&A for any given period, the book value is computed

a. By taking the beginning of period costs less beginning of period accumulated depreciation

b. By taking the end of period costs less beginning of period accumulated depreciation

c. By taking the costs as of the end of the prior period less beginning of period accumulated depreciation

d. By taking the costs as of the end of the prior period less accumulated depreciation as of the end of the prior period

e. None of these apply