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Noise with double-sided power spectral density of $\mathrm{K}$ over all frequencies is passed through a RC low pass filter with $3 \mathrm{~dB}$ cut-off frequency of $f_{c}$. The noise power at the filter output is

1. $\mathrm{K}$
2. $\mathrm{K} f_{c}$
3. $\mathrm{K} \pi f_{c}$
4. $\infty$