Reading comp PrepTest 137 · Section 1 · Question 11

Incorrect. There's no statement in the stimulus strong enough to allow an inference about "[m]ost" diamonds. While diamonds with that much S33 must have formed when there was little to no oxygen in Earth's atmosphere, we have no idea how long that was true—it was true 2.9b years ago, but it may have persisted until 500m years ago, leaving open 2.4b years for diamonds to form.

Incorrect. There's absolutely no evidence anywhere in the stimulus that suggests this answer.

Correct. Argument or Facts:
Facts

Question Type:
Must Be True

Stimulus Summary:
Diamonds from 2.9m years ago had higher-than-normal S33
Higher-than-normal S33 → Reactions stimulated by UV light
More than trace oxygen → not Reactions stimulated by UV light

Answer Anticipation:
This stimulus features a couple conditional statements ("only", "If") that share a term, and a nonconditional statement that also overlaps with the conditionals. As such, it's likely that we're going to have to combine the conditionals and then check to see if the nonconditional statement triggers that chain, thus letting us draw an inference.

We'll need to take the contrapositive of one of the conditionals since the shared term is on the same side of the arrow but negated, but it doesn't matter (logically) which one. Since the nonconditional statement establishes the sufficient condition of the first conditional, let's take the contrapositive of the second:

Reactions stimulated by UV light → not More than trace oxygen

With that taken, we can see the two conditionals chain together. Since the stimulus establishes through the diamonds that the first conditional is triggered, we know that there were reactions stimulated by UV light 2.9 billion years ago, which means—according to the contrapositive of the second—that there wasn't more than trace levels of oxygen in the atmosphere. Let's find that answer.

Answer Explanation:
If the high level of S33 in the diamonds shows that UV light was stimulating certain reactions, and that guarantees there wasn't more than trace amounts of oxygen in the Earth's atmosphere, then 2.9 billion years ago there wasn't much oxygen in Earth's atmosphere, as this answer states.

Key Takeaway:
A stimulus that has overlapping conditional and nonconditional statements will likely chain the conditionals together and have the nonconditional trigger the chain. In that case, the most likely correct answer is the necessary condition of the chain.

Incorrect. S33 wouldn't be found in diamonds formed when there's more than trace oxygen in the atmosphere, but we don't know what time period that covers. While 2.9b years ago definitely falls into the relevant time period, it could have continued for another couple billion years.

Incorrect. The stimulus only discusses diamonds formed with higher-than-normal S33 levels, and we don't know what percentage of diamonds that makes up, so there's no way to know the conditions under which all ("only in") diamonds form.