Answer: option D. 4
Reasoning:
Assuming that the time elapsed between any two consecutive dots is the same, and that the second dot marks second 1, the column with 3 dots marks second 2, and the dot right most marks second 3, the motorcycle has positive acceleration after 4 seconds.
That is because:
1) from the first dot to the second dot: the velocity the lenght of the arrow that models the velocity decrases, which means that the acceleration is negative.
2) In the nex group of dots the length of the arrow remains equal, which means that the velocity is constant and the acceleration is 0.
3) For the last dot, the length of the arrow is greater, which means that the velocity increases, and so the acceleration is positive.
