Your friend promises you a perpetuity of $1 every year, which starts in year 1. However, your friend is an absent-minded guy, paying you $3 at year 8 but no payment at years 7 and 9. Except for these three years, in other years, the payment is always $1. Which of the following is right about the present value at year zero of your friend's all payments if r = 5%? Select one: a. $21.09 b. $20.00 c. $19.91 d. $19.97 e. None of the above