If f(x) is defined on [0,1] as f(x) = x, x ∈ Q; f(x) = -1 - x, x ∉ Q, then:
a) f(f(x)) = x
b) f(f(x)) = -1 - x
c) f(f(x)) = 1 - x
d) f(f(x)) = -x