A signal $x(t)=100 \cos \left(24 \pi \times 10^{3}\right) t$ is ideally sampled with a sampling period of $50 \; \mu \mathrm{sec}$ and then passed through an ideal lowpass filter with cut-off frequency of $15 \; \mathrm{kHz}$. Which of the following frequencies is/are present at the filter output?
- $12 \; \mathrm{kHz}$ only
- $8 \; \mathrm{kHz}$ only
- $12 \; \mathrm{kHz}$ and $9 \; \mathrm{kHz}$
- $12 \; \mathrm{kHz}$ and $8 \; \mathrm{KHz}$