Samuel contributed \($10,000\) a year to his retirement plan from his before-tax earnings (that is, the contribution was not included in his taxable wages). His employer contributed \($5,000\) a year to Samuel’s retirement fund. After 30 years of contributions, Samuel retires and receives a distribution of \($900,000,\) the balance in his retirement fund. Samuel must include what amount in gross income?

a. $0

b. $450,000

c. $600,000

d. $900,000