\textbf{191-} A transverse wave with speed $100\,\dfrac{\text{m}}{\text{s}}$ and frequency $50\,\text{Hz}$ is propagating in a medium. The phase change of a point on the medium over a time interval of $2.5$ milliseconds is how many radians?
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(1) $\pi$ \hspace{2cm} (2) $\dfrac{\pi}{2}$ \hspace{2cm} (3) $\dfrac{\pi}{4}$ \hspace{2cm} (4) $\dfrac{\pi}{8}$
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