'P' is the investment made on an equipment, 'S' is its salvage value and 'n is the life of the equipment in years. The depreciation for Rath year by the sum-of years digit method will be

A. (P - S)/n

B. 1 - (P/S)1/m

C. (m/n) (P - S)

D. [2 (n - m + 1)/n(n + 1)]. (P - S)

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