A square wave signal of frequency $20$ $\mathrm{kHz}$ is passed through an ideal low-pass filter with cut-off frequency of $21$ $\mathrm{kHz}$. The output signal is a $\_\_\_\_\_\_$.
- $20$ $\mathrm{kHz}$ sine wave
- $20$ $\mathrm{kHz}$ square wave
- $20$ $\mathrm{kHz}$ triangular wave
- $20$ $\mathrm{kHz}$ saw-tooth wave
