The set of all values of a for which the function f(x) = (√a+4/1-a -1)x⁵-3x + log5 decreases for all real x is

a. (-[infinity],[infinity])
b. [-4, 3-√21/2] ∪ (1,[infinity])
c. (-3.5 - √27/2) ∪ (2,[infinity])
d. (1,[infinity])