AMC 8 2011 Problem 9

Carmen takes a long bike ride on a hilly highway. The graph indicates the miles traveled during the time of her ride. What is Carmen's average speed for her entire ride in miles per hour?

$\textbf{(A)}2 \textbf{(B)}2.5 \textbf{(C)}4 \textbf{(D)}4.5 \textbf{(E)}5$