An electromagnetic wave with frequency $\omega$ and wavelength $\lambda$ travels in the $+y$ direction. Its magnetic field is along the $+x$ axis. The vector equation for the associated electric field (of amplitude $E_0$) is:

• A
  $\vec{E} = E_0 \cos \left( \omega t - \frac{2\pi}{\lambda} y \right) \hat{x}$
• B
  $\vec{E} = -E_0 \cos \left( \omega t + \frac{2\pi}{\lambda} y \right) \hat{x}$
• C
  $\vec{E} = -E_0 \cos \left( \omega t - \frac{2\pi}{\lambda} y \right) \hat{z}$
• D
  $\vec{E} = E_0 \cos \left( \omega t - \frac{2\pi}{\lambda} y \right) \hat{z}$