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The electric field component of a time harmonic plane EM wave traveling in a nonmagnetic lossless dielectric medium has an amplitude of $1 \; \mathrm{V/m}$. If the relative permittivity of the medium is $4$, the magnitude of the time-average power density vector (in $\text{W/m}^{2}$ ) is

- $\frac{1}{30 \pi}$
- $\frac{1}{60 \pi}$
- $\frac{1}{120 \pi}$
- $\frac{1}{240 \pi}$