Only part c. please explain.

if you can continue to earn interest at a rate of: 5% 8% 10% 11% (Length of an ordinary annuity) You plan to make deposits into a retirement account and have set a goal of having $2 million in the account when you retire. You forecast you can earn interest at a 6% rate. How many years will it take for you to reach your retirement goal if, at the end of each year, you deposit: $10,000? $20,000? $30,000? $40,000