Select all of the following equation(s) that are quadratic in form. a) x4 – 6x2 – 27 = 0

b) 3x4 = 2x

c) 2(x + 5)4 + 2x2 + 5 = 0

d) 6(2x + 4)2 = (2x + 4) + 2

e) 6x4 = -x2 + 5

f) 8x4 + 2x2 – 4x = 0