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The input and output of a continuous time system are respectively denoted by $x(t)$ and $y(t)$. Which of the following descriptions corresponds to a causal system?

1. $y(t)=x(t-2)+x(t+4)$
2. $y(t)=(t-4) x(t+1)$
3. $y(t)=(t+4) x(t-1)$
4. $y(t)=(t+5) x(t+5)$