This is the essence of search — choosing one option and putting the others aside for later, in ‘ case the first choice does not lead to a solution. Suppose we choose Zerind. We check to see if it is a goal state (it is not), and then expand it to get "in Arad" and "in Oradea." We can then choose
any of these two, or go back and choose Sibiu or Timisoara.
We continue choosing, testing, and expanding until a solution is found, or until there are no more states to be expanded.
The choice of which state to expand first is determined by the search strategy.