Alpha and Beta are twin cities having a common airport. Alpha City is entrusted with the development work of the airport, the cost of which will be shared by Beta City. Alpha City completes the job and sends the 50% bill to Beta City for development of the airport as well as the road network from the two cities. Beta City is, however, reluctant to foot the 50% bill although there was a prior understanding that 50% of the airport development cost will be borne by the city.

Which of the following provides the best reason for the stand of Beta City?