A plane electromagnetic wave travels through a medium and the magnetic field associated with it is given by $B = 5 \times 10^{-8} \sin (3 \times 10^{10} t - 150 x) \text{ T}$ where x is in metres and t is in seconds. The velocity of the wave is : (A) $2.0 \times 10^8 \text{ m s}^{-1}$ (B) $4.5 \times 10^7 \text{ m s}^{-1}$ (C) $3.5 \times 10^7 \text{ m s}^{-1}$ (D) $2.5 \times 10^8 \text{ m s}^{-1}$