A rectangular pulse train $s(t)$ as shown in the figure is convolved with the signal $\cos ^{2}\left(4 p \times 10^{3} t\right)$. The convolved signal will be a
- $\text{DC}$
- $12 \; \mathrm{kHz}$ sinusoid
- $8 \; \mathrm{kHz}$ sinusoid
- $14 \; \mathrm{kHz}$ sinusoid