Answer:
cross out the false piece in blue and write the true piece in red
A) the sound waves were pushed closer together
Impulse = Force * times and also Impulse = change in momentum.
Given that the mass does not change, change if momentum = mass * (final velocity - initial velocity)
Given that you know mass and initial velocity (which is the velicity before the cart hits the wall) you need the final velocity (which is the velocity after the cart hits the wall).
Answer: the velocity of the cart after it hits the wall.
