What is the present value of the following annuities?
a. $ 2,500 a year for 10 years discounted back to the present at 7 percent
b. $ 70 a year for 3 years discounted back to the present at 3 percent
c. $ 280 a year for 7 years discounted back to the present at 6 percent
d. $ 500 a year for 10 years discounted back to the present at 10 percent