(Present value of an annuity) What is the present value of the following annuities? a. $2,800 a year for 10 years discounted back to the present at 11 percent. b. $80 a year for 4 years discounted back to the present at 9 percent. c. $250 a year for 11 years discounted back to the present at 13 percent. d. $470 a year for 5 years discounted back to the present at 5 percent