Derive the forced response of the first-order system y' + ay = f(t), with the pulse input f(t) = {0, 0 ≤ t < 0; 1, t ≥ 0}.
a) Integral of e^(-at) from 0 to t
b) 1 - e^(-at)
c) ae^(-at)
d) 1 - ae^(-at)