So we need to find the average speed of the bus that travels 20 km in 30 minutes. Average speed is defined as: Average speed=distance travelled/time taken or: v=s/t. We know s=20 km and t=30 minutes=0.5h. So after we input our numbers in the equation, we get: v=20km/0.5h=40km/h. So the correct answer is C) 40km/h.