\textbf{57.} According to the figure, a string fixed between two supports oscillates at its first harmonic with frequency $f$. If the distance between the two supports is $50\,\text{cm}$ and the transverse wave speed in it is $25\,\dfrac{\text{m}}{\text{s}}$, how many milliseconds does it take for each particle of the string to complete one oscillation?
\textit{[Figure: string fixed at both ends vibrating in first harmonic mode]}
\begin{itemize}
\item[(1)] 25
\item[(2)] 2
\item[(3)] 5
\item[(4)] 4
\end{itemize}