The equation of a transverse wave travelling along a string is \( y(x,t) = 4.0 \sin \big( 20 \times 10^{-3} x + 600 t \big) \, \text{mm} \), where \( x \) is in \( \text{mm} \) and \( t \) is in second. The velocity of the wave is:
\( -30 \, \text{m/s} \)
\( +60 \, \text{m/s} \)
\( +30 \, \text{m/s} \)
\( -60 \, \text{m/s} \)