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

$X$-rays,gamma rays and microwaves travelling in vacuum have

The electric field part of an electromagnetic wave in a medium is represented by $E_x=0, E_y=2.5 \frac{N}{C} \cos \left[ (2\pi \times 10^6 \frac{rad}{s})t - (\pi \times 10^{-2} \frac{rad}{m})x \right]$,and $E_z=0$. The wave is

The electric field for an electromagnetic wave in free space is $E = i 30 \cos (k z - 5 \times 10^8 t)$,where the magnitude of $E$ is in $V/m$. The magnitude of the wave vector $k$ is (velocity of electromagnetic wave in free space $= 3 \times 10^8 \ m/s$).

What is the range of frequency of $EM$ waves that are reflected back by the ionosphere?

In a plane electromagnetic wave,which of the following has/have a zero average value in one complete cycle?
$(a)$ Magnetic field
$(b)$ Magnetic energy
$(c)$ Electric field
$(d)$ Electric energy