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A sinusoid $x(t)$ of unknown frequency is sampled by an impulse train of period $20$ $ms$. The resulting sample train is next applied to an ideal lowpass filter with a cutoff at $25$ $Hz$. The filter output is seen to be a sinusoid of frequency $20$ $Hz$. This means that $x(t)$ has a frequency of

  1. $10$ $Hz$
  2. $60$ $Hz$
  3. $30$ $Hz$
  4. $90$ $Hz$
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