High frequency photography

The upper trolley moves in a straight line at a uniform speed, and the lower trolley moves in a straight line at a uniform speed;

Set the width s of each cell on the negative, and the corresponding time is t;

The speed of the car above: v=4St

According to the average speed at the intermediate moment is equal to the instantaneous speed, for the following cars, there are:

Speed at t2: v2 = 5s2t < v；;

Speed at t3: v3 =14s4t = 7s2t < v;

Speed at t4: v4 = 9s2t > v；;

So the time when the speed is equal appears at some time between t3 and t4, so ABC is wrong and D is right;

Therefore, choose: d.