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Here are my homework questions and my answers:

Are the systems described by the following equations with input x(t) and output y(t) causal or noncausal?

A) y(t)=x(t-2) : CAUSAL; involves a delay of the signal in real time.

B) y(t)=x(-t) : NONCAUSAL; causal signals do not exist before t=0 (I think?).

C) y(t)=x(at), a>1 : CAUSAL; a is never negative.

D) y(t)=x(at), a<1 : CAUSAL; a may be any real number between 0 and 1.

Can anyone verify or correct me on any of these?