What is the average acceleration vector given a car's north and east velocities?

[lostpersson]

Question: A car is moving north at 100km/h. An hour later it is moving east at 100km/h. Its average acceleration during this hour is:
a) A vector pointing northeast
b) A vector pointing southeast
c) A vector pointing southwest
d) A vector pointing east
e) Zero

Attempt: I confidently chose a) A vector pointing northeast, but apparently that is wrong. The correct answer is b) A vector pointing southeast. I don't understand why this is so, since when you add the two vectors tip to tail, the resultant vector is pointing northeast...

 

[TSny]

Does the average acceleration involve the addition of the two velocity vectors or the subtraction of them? Might be a good idea to review the definition of average acceleration.