Answer:
b flashback, c characters, d setting
The answer is: creating an analogy.
An analogy is a comparison between two things, based on their structure and for explanatory or clarification purposes.
In the excerpt of St. Thomas Aquinas, the author Chesterton provides a contrast between finding a corner in a curve, and concludes that there is no such thing. As a result, even if the curve is placed upside down and it looks upwards instead of downwards, there is no peak point in the curve.
