Read the excerpt below and answer the question that follows. They had worked the trip out with a pencil, so much to a dog, so many dogs, so many days, Q. E.D. Mercedes looked over their shoulders and nodded comprehensively, it was all so very simple.
In the passage above, we can interpret the phrase "it was all so very simple" as being an example of subtle .

sincerity
sarcasm
surprise
pride