A plane electromagnetic wave of frequency $f=50\text{ MHz}$ travels in free space along the positive $x$-direction. At a particular point in space and time, the electric field is $\vec{E}=6.3\hat{j}\text{ V/m}$. What is the corresponding magnetic field $\vec{B}$ at this point?

1. A.$2.1\times 10^{-8}\text{ T}$ along the $z$-direction
2. B.$2.1\times 10^{-8}\text{ T}$ along the negative $z$-direction
3. C.$1.89\times 10^{-9}\text{ T}$ along the $z$-direction
4. D.$1.89\times 10^{-9}\text{ T}$ along the negative $z$-direction