A company has invested significant funds into building a new subway line in City A$.$ A risk analysis conducted by a research institute recently estimated that the project may experience cost overruns by up to $$5$ billion, which is more than originally estimated. The CEO of the company needs the Board to approve additional funding to finish the project. Indicate whether the CEO$$s statement is economically valid or invalid. Explain your answer if you think the statement is invalid.

a$.$We have already invested a significant amount of time and money, so we cannot abandon the project now.

b$.$We should continue with the project because the expected benefits to finish it exceed the remaining costs.$$