There are three v$\text{'}$s in the doppler equation. v$,$ Vs$,$ and vo$.$ Suppose a train is moving away from a stationary woman at a speed of $25$ m$/$s on a normal day. The train sounds a horn whose frequency is $200$ Hz$,$ but it sounds lower to the woman. Which of the following would be an appropriate value for v$?$a$)0$ m$/$s b$)25$ m$/$sc$)200$ m$/$sd$340$ m$/$s