\textbf{69.} An ideal solenoid without a core, with length $15.7$ cm, has $1000$ turns. If the cross-sectional area of each turn is $8\,\text{cm}^2$, what is its inductance in millihenries? $\left(\mu_0 = 4\pi \times 10^{-7}\,\dfrac{\text{T.m}}{\text{A}}\right)$
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(1) $6.4$ \hspace{2em} (2) $64$ \hspace{2em} (3) $1.6$ \hspace{2em} (4) $16$
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