The thing which the detectives do when they spot William is that they chase after him.
What is Narration?
This refers to the telling of a story and the sequence of actions that takes place in a story.
Hence, we can see from the complete text that there is the narration of the actions of the detectives and how they react when they spot Williams and because he has been on their radar for a long time, they chase him.
Read more about narration here:
https://brainly.com/question/1934766
