For an object to undergo a change in its momentum, a net force needs to be applied. The amount of change in momentum produced by the force depends on the length of the time during which the force acts and is called the impulse. That is,
$$
\text { Impulse }=\Delta \overrightarrow{\mathbf{p}}=\overrightarrow{\mathbf{F}} \Delta t
$$
where the force $\overrightarrow{\mathbf{F}}$ is assumed to be constant, or to be an "average force." For each of the collisions, calculate the average force acting on the cars during the collision, and compare them.