In HSK5级 应试指南 (pages 68 and 69), there's an exercise about Confucius (孔子). Two children are arguing about whether or not the sun is closer to the earth during the day. Confucius doesn't know how to answer, and from there...
The choices for the blank are:
C. 为什么你这么聪明呢 (✗)
D. 谁说你的知识很丰富呢 (✓)
The 笑着 indicates humor is involved in the answer. B is not a joke, so it's incorrect. A is like a genuine question, which is not correct since 孔子 didn't say anything.
I thought the answer was C. 为什么你这么聪明呢 = "Why are you so smart?". This reminds me of a typical joke where there is a group of people together without any of them speaking, and someone might ironically remark "Gee, don't all speak at once!" And indeed, a sign of intelligence is not claiming to know an answer when you don't.
However, the answer is D. 谁说你的知识很丰富呢 = "Who says you have plentiful knowledge?" To me, this sounds like a flat-out insult (claiming 孔子 is not as smart as people say), and not humorous.
The books explanation is as follows:
But it seems C, when said jokingly in the given context, also has a 否定的意思. There seems to be something here about Chinese-style humor I'm missing.
Question: Why do the students joke 谁说你的知识很丰富呢 and not 为什么你这么聪明呢?