Solve x4 - 17x2 + 16 = 0. Let u = x2. Rewrite the equation in terms of u.
a) u2 - 17u + 16 = 0
b) u2 + 17u + 16 = 0
c) u2 - 17u - 16 = 0
d) u2 + 17u - 16 = 0