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

If the magnetic field of a plane electromagnetic wave is given by $B = 5 \times 10^{-6} \sin (0.6 \times 10^2 x + 0.5 \times 10^{10} t)$,then the speed of the wave is

$A$ point source of electromagnetic radiation has an average power output of $800 \, W$. The maximum value of the electric field at a distance $4.0 \, m$ from the source is .... $V/m$.

The amplitude of the electric field in an electromagnetic wave is $1 \ V/m$. The frequency of the wave is $5 \times 10^{14} \ Hz$. The wave propagates along the $z$-axis. What is the average energy density of the electric field in $J/m^3$?

The biological importance of the ozone layer is that:

If the peak value of the magnetic field of an electromagnetic wave is $30 \times 10^{-9} \ T$,then the peak value of the electric field is (in $Vm^{-1}$)