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 $h_2(t)$ from $h_1(t)$