19. Luis started a new job and decided to open a bank account with $20. Each week he will deposit $12 from his pay to the account. Which of the following
recursive formulas can be used to model the total amount of money in Giovanni's
account?
a. an = an-1 + 12, where ai = 20
b. an = an + 20, where ai = 12
c. an = an + 12, where ai = 20
d. an = an-1 + 20, where ai = 12