(Present value of an annuity) 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