(Present value of an annuity) What is the present value of the following annuities? a. $2,500 a year for 10 years disconnected back to the present at 7%. b. $70 a year for 3 years disconnected back to the present at 3%. c. $280 a year for 7 years disconnected back to the present at 6%. d. $500 a year for 10 years disconnected back to the present at 10%.