Here's a problem to solve: An airplane flies in a straight line between two cities that are separated by 840 kilometers. If it takes 4 hours for the airplane to make the trip, what is its average speed?

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

- A: 84 km/h
 
- B: 105 km/h
 
- C: 180 km/h
 
- D: 210 km/h
 