Factor each polynomial function. Based on these factors, which polynomial’s graph crosses the x-axis at only one point? a. f(x)= x^3 - 3x^2 - 4x + 12, b. f (x)= x^3 + 2x^2 - 9x - 18 c. f (x)= x^3 - 2x^2 - 16 +32 d. f (x)= x^3 - 7x^2 + 3x - 21