Answer:D. The cash flows for an annuity must all be equal and they must occur at regular intervals, such as once a year or month . If some cash flows occurred at the beginning of the periods while others occurred at the ends, then we have what the text books defined as variable cash flow.
Explanation:
An annuity is a fixed amount of cash inflow or outflow receivable or payable periodically over a period of time. When the cash flows periods varies it's usually that it has a definite receipt period and the payment period is subject to the end of the receipt period e.g a pension scheme that is payed during work life but can only be received after retirement.