$A$ plane electromagnetic wave of frequency $25 \text{ MHz}$ propagates in vacuum along the positive $x$-direction. At a particular point in space and time,if the electric field is $6.3 \hat{j} \text{ Vm}^{-1}$,then the magnitude of the magnetic field of the wave at this point at the same time is:

• A
  $2.1 \times 10^{-8} \text{ T}$
• B
  $4.2 \times 10^{-8} \text{ T}$
• C
  $6.3 \times 10^{-8} \text{ T}$
• D
  $8.4 \times 10^{-8} \text{ T}$