Which of the following is $NOT$ true for electromagnetic waves?

Three observers $A, B$ and $C$ measure the speed of light coming from a source. Observer $A$ moves towards the source,observer $C$ moves away from the source with the same speed,and observer $B$ remains stationary. The surrounding space is a vacuum everywhere. Which of the following is true regarding the measured speeds ${v_A}, {v_B}$ and ${v_C}$?

In a plane electromagnetic wave,the average value of ....... is zero.

An electromagnetic wave having frequency $4 \times 10^{14} \,Hz$ is passing through a small volume. The energy contained in this volume oscillates with frequency

On a particular day,the sun delivers an average power of $\left(\frac{6}{\pi} \times 10^3\right) \frac{W}{m^2}$ to the top of Earth's atmosphere. Find the amplitude of the magnetic field for the electromagnetic waves above the atmosphere. (Take $\mu_0 = 4\pi \times 10^{-7} \text{ SI units}$)

The electric field of an electromagnetic wave travelling through a medium is given by $\overline{E}(x,t) = 25 \sin(2.0 \times 10^{15}t - 10^{7}x)\hat{n}$. The refractive index of the medium is . . . . . . . (All given measurements are in $SI$ units)