Which of the following statements is CORRECT? 0 a. The cash flows for an ordinary annuity all occur at the beginning of the periods O b. If a series of unequal cash flows occurs at regular intervals, such as once a year, then the series is by definition an annuity C. The cash flows for an annuity due must all occur at the beginning of the periods. O d. cash flows for an annuity may vary from period to period, but they must occur at regular intervals, such as once a year or once a month O e. If some cash flows occur at the beginning of the periods while others occur at the ends, then we have what the textbook defines as a variable annuity