In an electromagnetic wave,what has an average value of zero?

$A$ plane electromagnetic wave of wavelength $3.0 \ m$ travels in vacuum along the positive $X$-axis. The electric field of amplitude $300 \ Vm^{-1}$ oscillates parallel to the $Y$-axis. Then the intensity of the wave is $(\mu_0 = 4\pi \times 10^{-7} \ Hm^{-1}, c = 3 \times 10^8 \ ms^{-1})$ (in $Wm^{-2}$)

The velocity of electromagnetic waves is parallel to which of the following?

The electric field in an electromagnetic wave is given as $\vec{E} = 20 \sin \omega (t - \frac{x}{c}) \hat{j} \text{ N/C}$. Where $\omega$ and $c$ are the angular frequency and velocity of the electromagnetic wave,respectively. The energy contained in a volume of $5 \times 10^{-4} \text{ m}^3$ will be $..... \times 10^{-13} \text{ J}$. (Given $\varepsilon_0 = 8.85 \times 10^{-12} \text{ C}^2/\text{Nm}^2$)

The velocity of an electromagnetic wave in a medium is $2.5 \times 10^{8} \ m/s$. What is the dielectric constant of the medium?

In the region far from the source,how are the oscillating electric and magnetic field vectors of an electromagnetic wave oriented?