A mast antenna consisting of a $50$ meter long vertical conductor operates over a perfectly conducting ground plane. It is base-fed at a frequency of $600-\mathrm{kHz}$. The radiation resistance of the antenna in ohms is
- $\frac{2 \pi^{2}}{5}$
- $\frac{\pi^{2}}{5}$
- $\frac{4 \pi^{2}}{5}$
- $20 \pi^{2}$