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?

The symbols, a and T, represent positive quantities, and u(t) is the u

1.	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?
A.	e+at u(t)
B.	e-a(t + T) u(t)
C.	1 + e-at u(t)
D.	e-a(t - T) u(t)
Answer» D. e-a(t - T) u(t)