An excitation is applied to a system at $$t = T$$ and its response is zero for $$ - \infty < t < T$$. Such a system is a

The impulse response and the excitation function of a linear time invariant casual system are shown in Fig. a and b respectively. The output of the system at t = 2 sec. is equal to

The response of an initially relaxed linear constant parameter network to a unit impulse applied at $$t = 0$$ is $$4{e^{ - 2t}}u\left( t \right).$$ The response of this network to a unit step function will be:

The transfer function of a zero-order hold is