Logical reasoning PrepTest 117 · Section 3 · Question 24

Incorrect. While any star with no lithium in its atmosphere must be less massive than the sun, there's no indication that any celestial object lacking lithium must be less massive than the sun. The stimulus discusses only a few celestial objects so this answer is too broad.

Incorrect. It's possible that such a celestial object doesn't have a nuclear furnace at all! Or it may have a non—fully functional nuclear furnace, like a brown dwarf.

Correct. Argument or Facts:
Facts

Question Type:
Must Be True

Stimulus Summary:
Brown dwarfs:
(1) More mass than planets
(2) Less mass than stars
(3) Identified by mass AND whether they have lithium

Stars with:
(1) The Sun's mass+ have lithium (mixing of elements in nuclear furnace incomplete)
(2) Less than the Sun's mass don't have lithium (mixing of elements in nuclear furnace complete)

Brown dwarfs pt. 2:
(1) No fully functioning nuclear furnace
(2) Lithium can't be consumed

Answer Anticipation:
Wow, that's a lot of science, and a lot of information. This argument is definitely trying to get you to get confused by that science or lost in the details.

However, there are definitely some aspects to this stimulus that we can focus on. First, it starts bringing up things about brown dwarfs before circling back to them. As such, the latter set of information is probably building off of the earlier information and the information about stars, so let's start there.

That section notes that brown dwarfs don't have fully functional nuclear furnaces. Which makes the information about stars with certain masses a bit out of scope. We could have determined that earlier since we know brown dwarfs aren't stars—and we can tell that because the stimulus compares them to stars (they're less massive than stars). So all that information about stars with more/less mass than the Sun doesn't apply to brown dwarfs.

Alright, well then, what do we learn is a result of that lack of a fully functional nuclear furnace? Brown dwarfs can't consume all the lithium present in them. We can add that to the information we already know about brown dwarfs, and it's time to head into the answer choices looking for something we can support based on that list of characteristics.

Answer Explanation:
Double negatives are there to throw you off! Be sure to rephrase this to remove the double negative—Any celestial object with no lithium in its atmosphere isn't a brown dwarf. We know that brown dwarfs "cannot . . . consume" their lithium so that lithium must persist in their atmosphere. As such any object with no lithium can't be a brown dwarf—as this answer states. This answer is the correct one and it is entirely derived from that last sentence!

Key Takeaway:
Correct answers in Must Be True questions don't need to use all statements from the stimulus. They don't even need to use multiple statements from the stimulus.

Incorrect. This is true of stars, but not of all celestial objects with lithium in their atmosphere. In fact, brown dwarfs have lithium and are less massive than stars—which would include the Sun.

Incorrect. We know that planets are celestial objects smaller than brown dwarfs, but we don't learn anything about lithium in their atmospheres, so we can't determine if this answer is true or false.