Logical reasoning PrepTest 137 · Section 3 · Question 10

Incorrect. The rule given is that a ring of gas couldn't maintain an orbit "so close," so it's possible that rings of gas with larger radii would still fall into the "too close" category. Additionally, this answer is a negation of the conditional rule, so it can't be inferred.

Incorrect. This ring of gas that's in a stable orbit around a black hole is emitting flickering X-rays, but there's no indication that other rings of gas not orbiting black holes don't emit flickering X-rays. There's no equivalently strong statement to back this answer up, so it's incorrect.

Correct. Argument or Facts:
Facts

Question Type:
Must Be True

Stimulus Summary:
Stem—Widely accepted physical theories are correct
There's a ring of gas orbiting around a black hole, and a widely accepted physical theory suggests the details mean this ring of gas is likely 49km
not Black hole spinning → not Ring of gas orbiting close to a black hole

Answer Anticipation:
First, note that the question stem provides us with a key piece of information—we should treat the widely accepted physical theories as correct. So even though that statement is qualified, here, we're not allowed to question whether the theory is correct.

With that said, we've got a Must Be True question that features a conditional rule, so we should see if we can apply it to the situation at hand. The rule talks about a black hole spinning or not, but there's no discussion of that earlier, so there's not a premise we can use to trigger that part of the conditional. However, we do know that the ring of gas is orbiting that close to a black hole, so information the stimulus has established triggers the contrapositive:

Ring of gas orbiting close to a black hole → Black hole spinning

This conditional allows us to conclude that the black hole this ring of gas is circling around must be spinning, because otherwise the 49km ring of gas we know to be orbiting it couldn't.

Answer Explanation:
If the widely accepted theories are correct, as we're told to treat them, then the ring of gas is pretty close to the black hole. The only way that can happen, according to that last statement in the stimulus, is if the black hole is spinning. Therefore, the black hole must be spinning, as this answer states.

Key Takeaway:
Must Be True questions frequently deal with conditional logic. When there's a single conditional rule presented, there's no opportunity to chain conditionals together to make inferences, so the correct answer will likely apply that conditional to some scenario presented in the stimulus (or, on rare occasions, in the answer choices).

Incorrect. There's no indication of a causal relationship between black holes spinning and rings of gas orbiting them—they're related solely by a conditional relationship in the stimulus. Conditional statements are correlations, not causal statements!

Incorrect. There's no indication that a black hole that isn't spinning is stationary. Ignoring that for a second, however, it's also possible that a stationary black hole (not spinning) isn't surrounded by a ring of gas at all—there's no indication these rings are necessary for black holes to exist!