How will the right answer fit in terms of support and conclusion?

Only the right answer will be a conclusion guaranteed by support in the passage.

Highlight the main conclusion in the passage, if there is one:

There is no conclusion in the passage. The right answer will be a valid conclusion.

[BACKGROUND]. [SUPPORT]. [BACKGROUND]. [BACKGROUND].

You won’t know for sure which parts are support and which parts are just background until you check the answers. So focus on eliminating any answers that bring in something new or use wording stronger than the passage uses.

Map the wording of the answers to the wording of the passage:

(A) The chimpanzee…evolved from the lemur.

This is pretty tricky, but the passage doesn’t support a conclusion this specific. You know “higher primates” evolved from a “diurnal species of lower primates”, but it says the lemur is the “only living” species like that. That means there could be an extinct species that chimps evolved from. So this can’t be “inferred” from the passage.

(B) No primates indigenous to Madagascar are diurnal higher primates.

The passage says the “only” primates indigenous to Madagascar are “lower primates”. If that’s true, then “No primates indigenous to Madagascar” are anything else. So this must be true.

(C) No higher primate is nocturnal.

You can’t make this conclusion just because “all higher primates” evolved from a diurnal species. That would mean using outside reasoning about how evolution works. That’s not allowed on the LSAT.

(D) There are some lemurs without opposable thumbs.

There’s no support for this in the passage. It only says “only primates have opposable thumbs”, but it doesn’t say whether all primates have thumbs.

(E) There are no nocturnal lemurs.

The passage only says “some species of lemurs” are diurnal, so there could totally be some nocturnal lemurs too. The passage doesn’t support this conclusion.

(B) is the correct answer.