A2. Let $f(x)=\frac{1}{\sqrt{a^{x}}}, a>0$ and $a \neq 1$. What is $f\left(\log _{a} 4\right)$?
(A) 2
(B) $-\frac{1}{2}$
(C) $\frac{1}{2}$
(D) -4
(E) -2
A2. We calculate $f\left(\log _{a} 4\right)=\frac{1}{\sqrt{a^{\log _{a} 4}}}=\frac{1}{\sqrt{4}}=\frac{1}{2}$.