One concept of income suggests that income be measured by determining the net change over time in the discounted present value of net cash flow expected to be received by the firm. Under this concept of income, ignoring income taxes, which of the following affect the amount of income for a period? A. The method used to depreciate property, plant and equipment B. Production of goods or services delivered last year C. Receiving cash from clients D. Windfall gains and losses due to external causes