A6. For which value of the number $a$ does the function $f(x)=\log _{4}\left(x^{2}+a\right)-2$ have a zero at $x=5$ ?
(A) -9
(B) 0
(C) 2
(D) 5
(E) 9

## II. PART
A6. We consider $0=\log _{4}(25+a)-2$. This simplifies to $2=\log _{4}(25+a)$, from which we get $16=25+a$ and finally $a=-9$.

II. PART