What is the present value of the following annuities? a. (5 pts.) $2.500 a year for 10 years discounted back to the present at 7 percent b. (5 pts.) $70 a year for 3 years discounted back to the present at 3 percent c. (5 pts.) $280 a year for 7 years discounted back to the present at 6 percent d. (5 pts.) \$500 a year for 10 years discounted back to the present at 10 percent