1. If you look at the units for the answers and look at the units given in the problem, you will notice that they don't match! By looking at the first answer (A), we know that this is incorrect because the units show velocity (displacement/time), not acceleration! To solve for the avg. acceleration (which is the change in velocity over the change in time) we can take the difference between the velocities and divide by 4 seconds. However, we must look at the units for this and figure out if they make sense. Without any unit conversions, we would have a = (50-40 km/hr)/4s = 10km/h*s. This looks close to answer B (unit-wise), however the magnitude is 10, not 5. So B isn't the answer either. This just leaves C and D as answer choices. To figure out if C is the correct answer, we must convert the units of km/h*s to km/s^2. So we take the answer for acceleration that we calculated and convert the units:
(10km/h*s) * (1h/3600s) = 0.00278km/s^2
which is also not a correct answer. By deduction, the correct answer would be D.
2. For this one, we just need to keep in mind the units for acceleration. We need a changing velocity over time to get acceleration. Answer A and C shows a change in displacement over time, which is velocity. Answer B however, shows a change in velocity over time, so this would be the correct answer!
