AMC 8 1990 Problem 23

The graph relates the distance traveled [in miles] to the time elapsed [in hours] on a trip taken by an experimental airplane. During which hour was the average speed of this airplane the largest?

$\text{(A)}\ \text{first (0-1)} \text{(B)}\ \text{second (1-2)} \text{(C)}\ \text{third (2-3)} \text{(D)}\ \text{ninth (8-9)} \text{(E)}\ \text{last (11-12)}$