Which of the following statements is CORRECT? 0 a. If some cash flows occur at the beginning of the periods while others occur at the ends, then we have what the ex book defines as a varable annut O b. The cash flows for an ordinary (or deferred) annuity all occur at the beginning of the periods. O c. The cash flows for an annuity must all be equal, and they must occur at regular intervals, such as once a year or once a month. O d. If a series of unequal cash flows occurs at regular intervals, such as once a year, then the series is by definition an annuity O e. The cash flows for an annuity due must all occur at the ends of the periods