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The input $x(t)$ and output $y(t)$ of a system are related as $y(t)=\underset{-\infty}{\int}x(\tau)\cos(3\tau)d\tau$. The system is

1. time-invariant and stable
2. stable and not time-invariant
3. time-invariant and not stable
4. not time-invariant and not stable