The set of all values of a for which the function f(x) = ((√a+4/1-a)-1)) x⁵ - 3x + log 5 decreases for all real x is
a. (-[infinity], [infinity])
b. [-4, (3√21)/2] ∪ (1, [infinity])
c. (-3.5 -√26/2) ∪ (2, [infinity])
d. (1, [infinity])