Logical reasoning PrepTest 144 · Section 4 · Question 19

Correct. Argument or Facts:
Facts

Question Type:
Must Be True

Stimulus Summary:
Oldest identified fragments of Earth's crust are microdiamonds from 4.2b years ago, which is only 300m years after the Earth's formation.

Answer Anticipation:
We stripped out a lot of the stimulus because it's unlikely to lead to a correct answer. For instance—it's hard to see how the 50 micron size could lead to an inference. Instead, we primarily highlighted to key phrases:

The microdiamonds are the "oldest fragments . . . yet identified" which is a strong comparison between these fragments and all others

The microdiamonds formed "only 300 million years" after the Earth, which compares their age

Since these are both comparisons, they're likely to play into the correct answer—especially since they overlap in setting the age. For instance, one inference here is that the Earth can't be more than 4.5b years old since these "oldest" fragments are 4.2b years old, and they formed only 300m years after the Earth formed. However, we should consider any answer about ages or timeline since that's what the comparisons are about.

Answer Explanation:
If the oldest known fragments of the Earth's crust are 300m years younger than the Earth itself, then the crust must have started to form by that time, thus supporting this answer.

Key Takeaway:
Some harder Must Be True questions are less about making inferences and more about not getting distracted by all of the extraneous information in the passage. When there are statements that don't seem like they could conceivably lead to an inference, and others that strongly hint that they do, focus on the latter. In general, strong statements, comparisons, causal statements, and conditionals will be relevant to the correct answer.

Incorrect. While these "oldest fragments" yet identified from the Earth's crust were found in Western Australia, it's possible that older fragments not yet identified will be found elsewhere, and that location is actually where the Earth's crust started to form. It's also possible that there are no fragments left of the Earth's earliest crust.

Incorrect. The stimulus lets us date the start of the Earth's crust's formation, but it doesn't speak to the end point of it, so this answer is out of scope.

Incorrect. While microdiamonds make up the oldest known fragments, we can't be sure that there aren't unknown fragments out there that are a different substance.

Incorrect. This answer seems too extreme. While microdiamonds are the earliest known components of the Earth's crust, there's no indication that they stopped forming when the Earth's crust stopped forming.