Self-study
Astronomers have long thought that the irregularity in the orbit of the planet Neptune was adequately explained by the gravitational pull exerted on Neptune by the planet Pluto. ███ ████ ██████ ████████████ ██ ██████ ████████ ████████ ████ ████ ██████ ██ ████ ███ █████ ██ █████ ███ ██████ ██ █████████████ ████ ██ ███████ ████ ███████████ ████ ███████ ██ ████
Stimulus Summary
The stimulus tells us about a belief long held by astronomers: that Neptune's irregular orbit can be "adequately explained" by Pluto exerting gravitational pull on Neptune. But more recent observations seem to undermine this belief, by showing that Pluto is too small to exert as much gravitational pull as it was once believed to.
Analysis: Potential Inferences
The more recent observations provide evidence against the astronomers' previous belief. Notice that this doesn't prove the astronomers' previous belief wrong. It could still be true that Pluto's gravitational pull explains Neptune's irregular orbit, and the irregularity is just caused by a much smaller pull than once thought. So we can't say it must be true that Pluto alone is not the cause of Neptune's irregular orbit.
But this isn't a Must Be True question. We're just looking for answer choices that are strongly supported. And for now, only going off the information in the stimulus, it's actually more supported to say that Pluto's gravitational pull alone no longer seems to "adequately explain" Neptune's orbit, and that some other factors might also be contributing to the irregularities of Neptune's orbit, than to insist that Pluto alone is the cause. So looking for answer choices saying something along those lines would be a good place to start.
13.
If the statements above are █████ ████ ███████ ███ ████ ███████ ███ █████ ███ ██ ███ ██████████
a
Neptune is somewhat ██████ ████ ██████████ ████ ████████ ██ ██ ███
Incorrect. Nothing in the stimulus suggests anything about the size of Neptune.
b
The orbit of ███████ ██ ████████████ ████ █████████ ████ ██████████ ████ ███████ ██ ████
Incorrect. Nothing in the stimulus suggests that scientists misunderstood the irregularity of Neptune's orbit.
c
There exists another, ██ ███ ████████████ ██████ ████ ██ █████ ██████ ████ ██ ██████
Incorrect. This is too specific. While the stimulus does suggest that there is some other cause for Neptune's irregular orbit besides Pluto's gravitational pull, nothing in the stimulus tells us that the cause is specifically another planet beyond Pluto.
d
The gravitational pull ██ █████ ██ ███ ███ ████ █████ ██ ███████████ █████████ ██████
Correct. The stimulus tells us that astronomers used to think that Pluto's gravitational pull "adequately explained" why Neptune's orbit was irregular. But now astronomers know that Pluto doesn't exert as much gravitational pull as they used to think. This supports the idea that there is another cause contributing to Neptune's irregular orbit besides Pluto's gravitational pull.
e
Further observations of █████ ████ ██████████ ████ ██ ██ ██ ████ ███████ ████ ██ ██ ███ ███████ ██ ███
Incorrect. Nothing in the stimulus suggests that Pluto will be discovered to be even smaller than currently thought.