Scooter C starts from rest, accelerates uniformly to 20m/s in 4s, and then maintains this speed for the next 2s. Scooter D travels at a steady 15m/s for the entire 6s. Taking the starting point as the origin, which scooter has the higher average speed over the 6s interval?
a) Scooter C only
b) Both have the same average speed
c) Scooter D only
d) The information is insufficient