We are looking for the answer choice that cannot be true (i.e. must be false).
Our conclusion is that: Cro-Magnons and Neanderthals, despite morphological differences, were members of the same species, and these differences are due merely to their having lived in different environments.
Why? Because the tools that these two groups of hominids, which lived in different environments, used were of exactly the same types. And we know that only if they faced the same daily challenges and met them in the same way would they have used such similar tools.
So: if they used such similar tools, then they faced the same daily challenges.
Well, we are told that they did use such similar tools. Therefore, we know that they faced the same daily challenges, despite living in different environments.
Thus, answer choice (B)--the daily challenges with which an environment confronts its inhabitants are unique to that environment--cannot be true, since we know from the passage that despite living in different environments, the fact that Cro-Magnons and Neanderthals used the same tools means that they faced the same daily challenges.
Hope that clears things up! Please let us know if you have any other questions.
