Here's a problem to solve: 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?

Among the 4 following options, which is the correct answer?

- A: 0.2 km/h^2
 
- B: 3.0 km/h^2
 
- C: 6.0 km/h^2
 
- D: 12.0 km/h^2
 