Define f(x)=x4−4x2 for all real x. Let S be the set of real numbers y for which (f−1)′(y) fails to exist. Which statement about S is true?
a) S contains exactly 3 distinct real numbers.
b) S contains exactly 2 distinct real numbers.
c) S contains exactly 1 distinct real number.
d) S contains infinitely many real numbers.