Two systems with impulse responses $h_1$(t) and $h_2$(t) are connected in cascade.then the overall impulse response of the cascaded system is given by
1. Product of $h_1$(t) and $h_2$(t)
2. Sum of $h_1$(t) and $h_2$(t).
3. convolution of $h_1$(t) and $h_2$(t).
4. subtraction of $h_1$(t) and $h_2$(t).