Here's a problem to solve: A bus traveled 280 kilometers between two cities. It left the first city at 3:00 p.m. and arrived at the second city at 7:00 p.m. What was the average speed of the bus during the trip?

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

- A: 4 km/h
 
- B: 40 km/h
 
- C: 70 km/h
 
- D: 280 km/h
 