College

What substitution should be used to rewrite [tex]$4x^4 - 21x^2 + 20 = 0$[/tex] as a quadratic equation?



A. [tex]u = x^2[/tex]



B. [tex]u = 2x^2[/tex]



C. [tex]u = x^4[/tex]



D. [tex]u = 4x^4[/tex]