Which of the following statements about annuities are true?
a. Ordinary annuities make fixed payments at the end of each period for a certain time period.
b. An annuity due is an annuity that makes a payment at the end of each period for a certain time period.
c. A perpetuity is a constant, infinite stream of equal cash flows that can be thought of as an infinite annuity.
d. An annuity due earns more interest than an ordinary annuity of equal time.