21 Given that the eccentricities of a hyperbola, an ellipse, and a parabola are the roots of a cubic equation, then this cubic equation is
A. $2 x^{3}-7 x^{2}+7 x-2=0$
B. $2 x^{3}+7 x^{2}-7 x-2=0$
C. $x^{3}-x^{2}+x-1=0$
D. $x^{3}+x^{2}-x-1=0$
21 A. Factor each equation.

保留了源文本的换行和格式。