Calculate acceleration of B during the first 20 s

Instruction: You are offered the test items on the base of text with one correct answer from four proposed ones. Read the text attentively and do the items.

Two cars

The graph shows the variation of velocity with time of two cars, A and B, which are travelling in the same direction over a period of time of 40 s. Car A, travelling at a constant velocity of 40 m/s, overtakes car B at time t = 0. In order to catch up with car A, car B immediately accelerates uniformly for 20 s to reach a constant velocity of 50 m/s.

[Graph showing velocity vs time for two cars, A and B. Car A has a constant velocity of 40 m/s. Car B starts at 25 m/s, accelerates uniformly to 50 m/s by 20 s, and then maintains 50 m/s until 40 s.]