Question: Two bicyclists were riding their bikes at a rate of 20 kilometers per hour (km/h). During the next half hour, they observed that they had increased their speed to 26 km/h. What was their average acceleration?

  Options: 
 A. 0.2 km/h^2
 B. 3.0 km/h^2
 C. 6.0 km/h^2
 D. 12.0 km/h^2 
Answer: D