Which of the following statements is CORRECT?

a$.$ The cash flows for an ordinary $($or deferred$)$ 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 ends of the periods.

d$.$ 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.

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.