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Let $x(t)= \cos(10\pi t)+\cos(30\pi t)$ be sampled at $20 Hz$ and reconstructed using an ideal low-pass filter with cut-off frequency of $20 Hz$. The frequency/frequencies present in the reconstructed signal is/are

1. $5$ Hz and $15$ Hz only
2. $10$ Hz and $15$ Hzonly
3. $5$ Hz, $10$ Hz and $15$Hz only
4. $5$ Hz only

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