\textbf{182.} A capacitor with capacitance $5\,\mu\text{F}$ has air between its plates. We want to change the plate separation without changing the charge on the plates. When the capacitor is charged with an electric potential difference of 20 V, the energy stored in it is 2 millijoules. The dielectric coefficient of the insulator is how much?
\begin{itemize}
\item[(1)] $1.5$
\item[(2)] $2$
\item[(3)] $2.5$
\item[(4)] $5$
\end{itemize}
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