Let h(t) be the impulse response of a linear time invariant system. Then the response of the system for any input u(t) is

A. $$\int\limits_0^t {h\left( \tau \right)} u\left( {t - \tau } \right)d\tau $$

B. $${d \over {dt}}\int\limits_0^t {h\left( \tau \right)} u\left( {t - \tau } \right)d\tau $$

C. $$\int\limits_0^t {\left[ {\int\limits_0^t {h\left( \tau \right)} u\left( {t - \tau } \right)d\tau } \right]} dt$$

D. $$\int\limits_0^t {{h^2}\left( \tau \right)} u\left( {t - \tau } \right)d\tau $$

Answer: Option A