(Here for the original audio source (MP3 file). The part in question begins approximately at 2'18'')
This conversation is an excerpt from one listening material in a TPO (TOEFL Practice Online) test, which takes place in an ecology lecture about phosphorus cycle:
Professor -...And the Phosphorus is recycled back into the soil; same thing with the animals that eat those plants, or eat other animals that have eaten those plants. We call all of this the land phase of the Phosphorus cycle. But a lot of the Phosphorus in the soil gets washed away into rivers by rain and melting snow. And so begins another phase of the cycle. Can anyone guess what it is called? Nancy?
Nancy -Well, if the one is called the land phase, then this has to be called the water phase, right?
Professor -Yes, that's such a difficult point isn't it? In a normal water phase, rivers eventually empty into oceans, and...
I was asked of what the professor implied when she said the sentence in bold text- "that's such a difficult point isn't it?". The options were
A. She realizes that the students are struggling with the concept.
B. She is surprised that the students knew the answer.
C. She thinks that the answer to the question is obvious.
D. She thinks that this phase of the cycle has an unusual name.
I felt slightly confused about the four options but wound up choosing B because I thought B could be inferred from what this sentence literally meant. However the answer was C.
So now I begin thinking it over, and one obvious explanation I can find is that the sentence doesn't tell what it literally means, i.e. it should have some implicit intention like humor or irony. But I cannot tell which one is what the speaker (the professor) truly intended. (It may be very easy to distinguish for a native ear, but for me it is not trivial.)
In accordance with the correct answer (C), humor should be the right interpretation. But then I am not able to explain why we cannot interpret the connotation alternatively, namely, as irony? I think irony can also explain the sentence in the context, and would of course lead to answer A.
What makes me even more confused is another detail which might be important: the professor seemed to say this sentence in a slightly laughing (or smirking) tone. Personally I cannot tell whether that was a sign of sarcasm or just for humorous purposes instead.
So which one do you think is a better interpretation, humor or irony?