A program consists of two modules executed sequentially. Let f₁(t) and f₂(t) respectively denote the probability density functions of time taken to execute the two modules. The probability density function of the overall time taken to execute the program is given by

A. $${{\text{f}}_1}\left( {\text{t}} \right) + {{\text{f}}_2}\left( {\text{t}} \right)$$

B. $$\int\limits_0^{\text{t}} {{{\text{f}}_1}\left( {\text{x}} \right){{\text{f}}_2}\left( {\text{x}} \right){\text{dx}}} $$

C. $$\int\limits_0^{\text{t}} {{{\text{f}}_1}\left( {\text{x}} \right){{\text{f}}_2}\left( {{\text{t}} - {\text{x}}} \right){\text{dx}}} $$

D. $$\max \left\{ {{{\text{f}}_1}\left( {\text{t}} \right),\,{{\text{f}}_2}\left( {\text{t}} \right)} \right\}$$

Answer: Option C