What is the main difference between an ordinary annuity and an annuity due?

a$.$ The ordinary annuity has payments due at the beginning of a payment period, whereas the annuity due payments are at the end.

b$.$ The annuity due has payments due at the beginning of a payment period, whereas the ordinary annuity payments are at the end.

c$.$ The ordinary annuity only allows for equal payments, whereas the annuity due can have payments that differ.

d$.$ The annuity due only allows for equal payments, whereas the ordinary annuity can have payments that differ.