#11 - Scientists examined diamonds that were formed on Earth

[Administrator]

Complete Question Explanation

Must Be True—SN. The correct answer choice is (C)

This stimulus presents a set of facts about research into the sulfur content of diamonds formed on Earth about 2.9 billion years ago. It turns out that the diamonds had a higher-than-normal concentration of sulfur-33. The stimulus offers facts to explain this result, but does not provide a conclusion.

First, we are told that only certain chemical reactions, stimulated by ultraviolet light, can account for the higher-than-normal concentration of sulfur-33 found in the diamonds. This is a conditional relationship, which we can diagram as:

HC = higher-than-normal concentration of sulfur-33
CR-UV = chemical reactions stimulated by ultraviolet light

• Sufficient Necessary
  
  HC CR-UV

Next, we are given another conditional relationship: “if there had been more than a trace of oxygen in Earth’s atmosphere 2.9 billion years ago, then not enough ultraviolet light would have reached Earth’s surface to stimulate the chemical reactions.” We can diagram this relationship as:

TO = more than a trace of oxygen in Earth’s atmosphere 2.9 billion years ago

• TO CR-UV

These two conditional relationships share a common term (CR-UV) that we can use to chain the relationships together and make an additive inference. First, though, we need to transform one of the statements to its contrapositive to permit the linkage, either making both terms “CR-UV” or making them “CR-UV.” If we were to transform the first conditional relationship, we would wind up with “HC” as our necessary condition. But, that term does not fit the stimulus, which was concerned with explaining the presence of the higher than normal concentration of sulfur-33 (HC), not its absence (HC).

So, using the contrapositive of the second relationship, we can form this conditional chain:

• HC CR-UV TO

This chain provides the additive inference that if diamonds have a higher-than-normal concentration of sulfur-33, then it must be the case that there was not more than a trace of oxygen in Earth’s atmosphere 2.9 billion years ago:

• HC TO

Not surprisingly, the question stem identifies this as a Must Be True question. Our prephrase is that the correct answer choice will ask about the conditional inference described above (i.e., HC TO).

Answer choice (A): The stimulus discussed only the chemical composition of certain diamonds, which were known to have been formed about 2.9 billion years ago. This answer choice goes beyond that evidence by making an inference about most diamonds with higher-than-normal concentrations of sulfur-33. The trick of this answer choice is to confuse you into thinking that the 2.9 billion year timeframe may be related to some form of cut-off, after which time there was more than a trace of oxygen in the atmosphere, so that diamonds with higher-than-normal concentrations of sulfur-33 could not be formed. However, the stimulus does not tell us when the atmosphere gained more oxygen.

Answer choice (B): This Shell Game answer choice takes various bits of the facts in the stimulus and jumbles them together in a way that is unsupported. An answer such as this can be confusing because it contains several relevant pieces of information, but is incorrect since it places those pieces into unsupported relationships.

Answer choice (C): This is the correct answer choice. This choice presents our prephrase, and reflects the additive inference described above.

Answer choice (D): The stimulus did not tell us anything about the chemical composition of diamonds formed more recently than 2.9 billion years ago, so this statement is without support.

Answer choice (E): This answer choice mistakes the role played in the stimulus by ultraviolet light, which stimulated chemical reactions necessary for the higher-than-normal concentrations of sulfur-33 in the diamonds. The stimulus did not state that the presence of ultraviolet light was necessary for the formation of diamonds generally.

[Sebastian]

So answer choice C states there was very little, IF any, oxygen 2.9B years ago. Wouldn't very little oxygen constitute the "more than a trace" aspect provided in the stimulus and thus not allow the chemical reaction to take place?

[Francis O'Rourke]

Hi Sebastian,

There are two points you are misinterpreting here.

In common language, we may say that "no more than a trace" means "not even a trace." However the LSAT means what it says. If there was only a trace of oxygen, or very little, then the chemical reaction may have occurred.

Secondly, answer choice (C) states that earth's atmosphere at the time contained very little, if any, oxygen. This means that it either contained none, or very little.

This phrase is a bit difficult to understand, but take a moment to consider it. When I tell you, for example I barely have a chance of making this basket, if I have any chance at all, I am not telling you that I have a chance. I am saying that I may have no chance, and that if I have a chance, it is barely any at all. I am in effect telling you that my chances of getting the ball in the basket are 0% or 1%. maybe 2%, but not much more than that.

So Choice (C) is telling us that if there was any oxygen at all, then it was very little. This fits the inference we can take from the stimulus.