111. Suppose $5^x = 10$ and $5^x = 20$. If $2^{f(x)} = 2$ holds, what is the function $f$? (1) $\dfrac{2x+1}{x+1}$ (2) $\dfrac{x-1}{2x-1}$ (3) $\dfrac{2x-1}{x-1}$ (4) $\dfrac{x+1}{2x+1}$
\textbf{111.} Suppose $5^x = 10$ and $5^x = 20$. If $2^{f(x)} = 2$ holds, what is the function $f$?
(1) $\dfrac{2x+1}{x+1}$ \qquad (2) $\dfrac{x-1}{2x-1}$ \qquad (3) $\dfrac{2x-1}{x-1}$ \qquad (4) $\dfrac{x+1}{2x+1}$