\textbf{68.} A coreless solenoid of length $6.28\ \text{cm}$ carries an electric current given by $I = 5\sin 100\pi t$ in SI units, and stores a maximum energy of 5 millijoules. If the cross-sectional area of each turn of the solenoid is $20\ \text{cm}^2$, how many turns does the solenoid have?
$$\left(\mu_o = 4\pi \times 10^{-7}\ \frac{\text{T.m}}{\text{A}}\right)$$
\begin{itemize}
\item[(1)] 500
\item[(2)] 400
\item[(3)] 300
\item[(4)] 100
\end{itemize}
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