11. E.

From Figure 3, we know that Naomi took 10 minutes to cover 6 miles, so her speed is $\frac{6}{\frac{1}{6}}=36$ miles per hour.
Similarly, Maya's speed is 12 miles per hour. Therefore, the average speed difference is $36-12=24$ miles per hour.
11. After school, Maya and Naomi head to a beach 6 miles away. Maya rides a bike, while Naomi takes the bus. Figure 3 shows the time and distance relationship during their trips. The difference in the average speeds of Naomi and Maya is ( ) miles/hour.
(A) 6
(B) 12
(C) 18
(D) 20
(E) 24