An astronaut drops a rock off the edge of a cliff on the Moon. The distance, d(t), in meters, the rock travels after t seconds can be modeled by the function d(t) = 0.8t^2 . What is the average speed, in meters per second, of the rock between 5 and 10 seconds after it was dropped? * 1 point
a) 12
b) 60
c) 20
d) 80