$A$ plane wave $y = A \sin \omega \left( t - \frac{x}{v} \right)$ undergoes normal incidence on a plane boundary separating medium $M_1$ and $M_2$ and splits into a reflected and transmitted wave having speeds $v_1$ and $v_2$. Then:

• A
  For all values of $v_1$ and $v_2$,the phase of the transmitted wave is the same as that of the incident wave.
• B
  The phase of the reflected wave depends upon $v_1$ and $v_2$.
• C
  The phase of the transmitted wave depends upon $v_1$ and $v_2$.
• D
  Both $(A)$ and $(B)$.