Answer:
D). Why the events in the story happen the way they do.
Explanation:
The essay 'I just couldn't put it down' relates a 'series of events' elaborating a step/event leading to another and so on. This chain of actions and events evoke the inquisitiveness of the reader and unable to 'put the book down'.
As per the question, the story's main focus is to exemplify that 'Why the events in the story happen the way they do' that explains the events in the story enhancing the curiosity of the readers by invoking their senses and they(readers) get more interested to read more and explore. Thus, option D is the correct answer.
