A signal x(t) = 100 cos $$(24\pi \times {10^3})$$ t is ideally sampled with a sampling period of 50 $$\mu \sec $$ and then passed through an ideal low pass filter with cutoff frequency of 15 KHz. Which of the following frequencies is/ are present at the filter output?

Which of the following cannot be the Fourier series expansion of a periodic signal?