The time averages Poynting vector, in $W / m^{2}$, for a wave with $\vec{E}=24 \; e^{} \; a_{} \mathrm{~V} / \mathrm{m}$ in free space is
- $ – \frac{2.4}{\pi} \vec{a}_{z}$
- $\frac{2.4}{\pi} \vec{a}_{z}$
- $\frac{4.8}{\pi} \vec{a}_{z}$
- $-\frac{4.8}{\pi} \vec{a}_{z}$