Which of the following statements is CORRECT?

A$.$ The cash flows for an annuity may occur at irregular intervals, as long as the amount is always the same.

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 ordinary annuity may occur at the beginning of the periods, as long as the amount is always the same.

D$.$ The cash flows for an ordinary annuity all occur at the end of the period.

E$.$ The cash flows for an annuity due must all occur at the end of the period.