In a series $\mathrm{RLC}$ circuit, $\mathrm{R}=2 \; \mathrm{k} \Omega, \mathrm{L}=1 \; \mathrm{H}$, and $\mathrm{C} \frac{1}{400}=\mu \mathrm{F}$. The resonant frequency is
- $2 \times 10^{4} \mathrm{~Hz}$
- $\frac{1}{\pi} \times 10^{4} \mathrm{~Hz}$
- $10^{4} \mathrm{~Hz}$
- $2 \pi \times 10^{4} \mathrm{~Hz}$