For a plane electromagnetic wave,which of the following quantities has an average value of zero?

Given below are two statements:
Statement $I$: Electromagnetic waves carry energy as they travel through space and this energy is equally shared by the electric and magnetic fields.
Statement $II$: When electromagnetic waves strike a surface,a pressure is exerted on the surface.
In the light of the above statements,choose the most appropriate answer from the options given below:

In a plane electromagnetic wave,$U_E$ and $U_B$ are the average energy densities of the electric field and magnetic field,respectively. Then,which of the following is the correct option?

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?

$A$ plane electromagnetic wave is given by $E_z = 100 \cos(6 \times 10^8 t + 4x) \text{ V/m}$. The refractive index of the medium in which the wave is traveling is:

$A$ sky wave with a frequency of $55 \text{ MHz}$ is incident on the $D$-region of the ionosphere at an angle of $45^\circ$. What is the angle of refraction? (The electron density in the $D$-region is $400 \text{ electrons/cm}^3$).