Which of the following statements is CORRECT?

a. The cash flows for an ordinary annuity all occur at the beginning of the periods.

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.

d. The 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.