Conclusion: Thomas must have seen the fire, whatever he now says to the contrary.
Why? We know that the fire that destroyed the Municipal Building started before dawn this morning and the last fire fighters did not leave until late this afternoon. We are told that "No ones could have been anywhere in the vicinity of a fire like that one and fail to notice it."
So, we are given a Sufficient & Necessary statement saying: If someone was anywhere in the vicinity of a fire like that, then they must have noticed it.
P1: IVF ==> MHN not MHN ==> not IVF
We are also told that Thomas has admitted that he went from his apartment to the library this morning, and there is no way for him to get from his apartment to the library without going past the Municipal Building. Thus, we can infer that this morning Thomas was in the vicinity of the fire, meaning, we can infer from "P1," that he must have noticed the fire.
So, the argument gives us a Sufficient & Necessary statement that tells us that being in the vicinity of the fire is sufficient to conclude that a person must have seen the fire.
Therefore, answer choice (B) is correct: "establishing that one thing occurred (Thomas must have seen the fire) by showing that another thing occurred (he was in the vicinity of the fire) and that this second thing (that he was in the vicinity of the fire) was enough to ensure the occurrence of the first thing (that he saw the fire)," i.e. we are told that being in the vicinity of the fire is sufficient to conclude that he saw the fire.
Hope that clears things up! Please let us know if you have any other questions.
