Which of the following series of cash flows is an annuity?

Select one:

a$.$ $$5,000$ to be received in one year, $$6,000$ to be received in two years, and $$4,000$ to be received in three years

b$.$ $$5,000$ to be received in one year, $$5,000$ to be received in two years, and $$5,000$ to be received in four years

c$.$ Both choices above are examples of annuities

d$.$ None of the choices above are correct