\textbf{71.} In a U-shaped tube with cross-sectional area $2\,\text{cm}^2$, mercury exists. In one of its arms, we pour alcohol so that the mercury in that arm, compared to the initial level, rises $0.5\,\text{cm}$ higher. How many cubic centimeters is the volume of the alcohol?
$$\left(\rho = 0.8\,\frac{\text{g}}{\text{cm}^3} \text{ for alcohol} \quad \text{and} \quad \rho = 13.6\,\frac{\text{g}}{\text{cm}^3} \text{ for mercury}\right)$$
\begin{itemize}
\item[(1)] $8.5$
\item[(2)] $17$
\item[(3)] $34$
\item[(4)] $51$
\end{itemize}
\vspace{0.5em}