Thanks for the question! Yes, I think you’ve got one part of it. Remember what the question is asking for: a hypothesis that is tested by a specific experiment described in the passage. (B) tells us that this hypothesis is that developing embryos initially grow many more nerve cells than they will eventually require. So there are two things wrong with this answer choice. The first: is this even a hypothesis? No, the passage presents this to us as the results of an experiment. The hypothesis may have matched the results, but it may not have; we can’t say for sure that this is even a hypothesis that was tested, and that’s one of the reasons (B) is wrong. The other reason is exactly what you mentioned: even if this were a hypothesis, the experiment itself wasn’t specifically described in the passage, and the question asks for a hypothesis that was tested by a specific experiment described in the passage. Telling us the results of an experiment is not the same thing as describing that experiment (for example, we’d expect to hear more about the methodology, the data analysis, etc. in a description). (A) is actually a hypothesis, and as you mentioned, its experiment is described in the passage, which makes it the correct answer choice here.
Hope this helps! Feel free to ask any other questions that you might have.
