189. Two coherent wave sources $S_1$ and $S_2$, in a homogeneous medium, emit waves and the wavelength is $25\,\text{cm}$. In this medium, the distance between the two sources $M$ from these two sources is $50\,\text{cm}$ and $80\,\text{cm}$ respectively. The phase difference between the two waves reaching point $M$ simultaneously, and the superposition of the two waves at this point, is:
\textbf{189.} Two coherent wave sources $S_1$ and $S_2$, in a homogeneous medium, emit waves and the wavelength is $25\,\text{cm}$. In this medium, the distance between the two sources $M$ from these two sources is $50\,\text{cm}$ and $80\,\text{cm}$ respectively. The phase difference between the two waves reaching point $M$ simultaneously, and the superposition of the two waves at this point, is:
\begin{itemize}
\item[(1)] $3\pi$, constructive \quad (2) $3\pi$, destructive \quad (3) $\dfrac{3\pi}{2}$, constructive \quad (4) $\dfrac{3\pi}{2}$, destructive
\end{itemize}