1. A ball rolls up a slope. At the end of three seconds its velocity is 20 cm/s; at the end of eight seconds its velocity is 0. What is the average acceleration from the third to the eighth second? A) 2.5 cm/s² B) 4.0 cm/s2 C) 5.0 cm/s2 D) 6.0 cm/s² E) 6.67 cm/s²