Signals and Systems Practice Test

The idea being tested is how causality and memory shape what inputs influence the output. A dynamic system has memory, so its output uses past inputs. A causal system cannot wait for future inputs, so it can only rely on inputs up to the present. Put together, the output at any moment depends on past inputs and the present input. That’s why past and present input values is the correct description.

Relying only on past inputs would exclude the present input, which many causal dynamic systems also use. Relying on present and future inputs or on all past, present, and future inputs would violate causality.