question_answer

The set of all values of 'a' for which the function \[f(x)=\left( \frac{\sqrt{a+4}}{1-a} \right){{x}^{5}}-3x+log\,5\] decreases for all real x is

A)  \[(-\,\infty ,\infty )\]

B)                     \[\left[ -4,\frac{3-\sqrt{21}}{2} \right]\cup (1,\infty )\]

C)  \[\left( -\,3,5-\frac{\sqrt{27}}{2} \right)\cup (2,\infty )\]

D)  \[[1,\infty )\]