# In a medium, in which a transverse progressive wave is travelling to the phase difference between two points with a distance of separation is $1.25\;cm$ is $\frac{\pi}{4}$. If the frequency of wave is $1000\;Hz$, its velocity will be :
( A ) $125 \; m/s$
( B ) $100\; m/s$
( C ) $10 \;m/s$
( D ) $10^4\;m/s$