Let the input be u and the output be y of a system, and the other parameters are real constants. Identify which among the following systems is not a linear system :

A. $$\frac{{{d^3}y}}{{d{t^3}}} + {a_1}\frac{{{d^2}y}}{{d{t^2}}} + {a_2}\frac{{dy}}{{dt}} + {a_3}y$$      $$ = {b_3}u + {b_2}\frac{{du}}{{dt}} + {b_1}\frac{{{d^2}u}}{{d{t^2}}}$$     (with initial rest conditions)

B. $$y\left( t \right) = \int\limits_0^t {{e^{\alpha \left( {t - \tau } \right)}}} \beta u\left( \tau \right)d\tau $$

C. y = au + b, b ≠ 0

D. y = au

Answer: Option C