Answer:
Car C increases it velocity
Step-by-step explanation:
From the problem statement, we can see that the two cars all along maintained the same velocities, even though car D was ahead, but at the said time, car C gained on car D, hence the velocity of car C increased more than that of car D, that was why the gap decreased.
A good way to exemplify this is to picture a friend stylishly waiting for you at a distance to meet with them, but they did not stop moving, they reduced their pace, you might have to increase your pace to meet with them, now you can observe the distance between you and them closes