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An automobile travels from city $A$ to city $B$ and returns to city $A$ by the same route. The speed of the vehicle during the onward and return journeys were constant at $60$ km/h and $90$ km/h, respectively. What is the average speed in km/h for the entire journey?

- $72$ km/h
- $73$ km/h
- $74$ km/h
- $75$ km/h