Three roots of a polynomial equation with real coefficients are 3, 5 – 3i, and −3i. Which of the following numbers must also be a root of the equation?

a. 5 + 3i only
b. −3 and 5 + 3i only
c. 5 + 3i and 3i only
d. −3, 5 + 3i, and 3i