From the momentum conservation we know that the initial momentum is equal to the final momentum. The momentum in a singular way can be defined as the product between the mass and the velocity of an object. In the presented system, however, there are two objects, therefore the mass of both and the speed of both, before and after the collision must be taken into account. Mathematically we could describe this as
Here,
= Mass of each object
= Initial velocity of each object
= Final velocity of each object
From here we can realize that it is necessary to use the system on both cars to be able to predict what will happen either with their masses, or their speeds.
The correct answer is C.