I'm working on homework, and I think I can explain the difference between causal and noncausal systems, but I don't know if I'm accurately recognizing them mathematically. 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?