A linear time-invariant, causal continuous time system has a rational transfer function with simple poles at s = - 2 and s = - 4, and one simple zero at s = - 1. A unit step u(t) is applied at the input of the system. At steady state, the output has constant value of 1. The impulse response of this system is

• A. [exp (- 2t) + exp (-4t)] u(t)
• B. [-4 exp (- 2t) + 12 exp (-4t) - exp (-t)] u(t)
• C. [-4 exp (- 2t) + 12 exp (-4t)] u(t)
• D. [-0.5 exp (- 2t) + 1.5 exp (-4t)] u(t)