The method of seeking common ground, also known as contract law, is one of the ways to explore the causal relationship between things. Its basic content is that in several occasions where the studied phenomenon A appears, only one antecedent situation is the same, then the same antecedent situation is the reason for the studied phenomenon A.
According to the above definition, the option 'The teeth of residents in some communities are very healthy, and residents also pay attention to the protection of teeth. Through investigation and study, it is found that there is natural fluoride in the water source of these communities, which shows that the fluoride in the water source is the reason why the teeth of the residents in these communities are healthy' belongs to the method of seeking common ground.

