# GATE Papers >> EEE >> 2019 >> Question No 17

Question No. 17

The symbols, $a$ and T, represent positive quantities, and $u(t)$ is the unit step function. Which one of the following impulse responses is NOT the output of a causal linear time-invariant system?

##### Answer : (C) $1+e^{-at}u\left(t\right)$

Solution of Question No 17 of GATE 2019 EEE Paper

If a L.T.I system is causal, we must should have the condition.

h(t) = 0; for t < 0

If we see the options a, c, d in these u(t) is multiplied that means their impulse response are zero for negative value of time, hence they are causal.

If we check option B

h(t) = 1 + e-at u(t)

We can see h(t) = 1; t < 0, hence it represents a non causal system.