Q13 - There are just two ways

[debbie.d.park]

I picked D thinking that between the two ways given in the stimulus, the second one applies to Earth's moon. Can you please explain why B is the correct answer? Much appreciated!

[ManhattanPrepLSAT1]

Okay. This is going to appear strange at first, but it's the most visual representation of the logic underlying the argument here.

ME ---> OS or IC
~OS
-------------------
~ME

(Notation Key: ME = made from the Earth, OS = outer shell spun off, IC = inner core dislodged)

This argument is testing contrapositive argument structure and rests on an assumption. We're simply asked to find the assumption.

To support the conclusion that the moon was not made from the Earth, we simply need to show that the moon is not made from dislodged material from the Earth's inner core. Best expressed in answer choice (B).

Answer choice (D) doesn't really establish that the Earth was struck by large meteoroids, and even if it did, that would undermine the conclusion, by suggesting that maybe the inner core was dislodged at some point in the past by one of those large meteoroids. This would suggest that maybe the moon was made from the Earth. The opposite of what we want to support.

Does that clear this one up?

[bixbee]

I had such a hard time understanding why I got this one wrong, until I finally realized I hadn't read the question stem or the argument closely enough!

They were looking to help justify the conclusion that the earth's moon was NOT formed from pieces of the Earth, and since they already established in the argument that it had not been created by the first of only two options for moons forming from their planets (from the outer shell), answer (B) helps rule out the second of the two possible explanations-that it was formed from a mass of material inside Earth as a result of a large object hitting the planet with enough force to dislodge a piece of inner Earth.

I had initially read the argument as there being only two ways, period, for a moon to be formed-not merely two ways for a moon to be formed from the planet around which it travels.

Similarly, I skimmed over the question stem, and incorrectly anticipated that they were looking for something to strengthen the idea that the moon was formed from a mass of material from inside the planet, which is why I chose (D).

Good lesson to read more carefully and not get ahead of myself!

[WaltGrace1983]

I liked this one so I'm gonna break it down for practice.

(Moon formed from Earth → Formed from outer shell or Formed from inner core)
+
~Formed from outer shell
→
~Moon formed from Earth

What is the problem with this argument? The argument is assuming that the other option didn't happen either. If we say, as this question does, that (FE → OS or IS) and we rule out OS, IS is still possible and unanswered for! It could very well be the case that FE didn't cause OS but definitely caused IS. The correct answer therefore MUST disprove IS, the idea that the moon was formed from the inner shell.

This is exactly what answer choice (B) does. If the elements of the moon are pretty different from what is in the Earth's core, we could most likely conclude that the the moon was (~Formed from the inner core). So what does this imply? What does this mean? Let me put the main premise up to show you.

(Moon formed from Earth → Formed from outer shell or Formed from inner core)

Well to recap, we have (~Formed from outer shell) from the premises and (~Formed from inner core) from the answer choice. Uh oh! We failed the necessary condition! and if we fail the necessary condition, then the sufficient condition cannot happen! This is shown in the contrapositive...

(~Formed from out shell and ~Formed from inner core → Moon ~formed from Earth

As for the incorrect answers...

(A) Cool. The Earth doesn't seem to be either!

(C) This actually weakens by eliminating an alternate cause of the Moon's formation. We want to say that the moon WASN'T formed from the Earth. By eliminating a possibility that the moon wasn't formed by Earth - but rather a meteoroid - then this gives more reason, albeit a weak reason, to believe that the Earth WAS formed by the Earth.

(D) Okay but we are talking about the EARTH. We don't care if the moon was struck. Get back to me when you are talking about the Earth.

(E) Once again, this is inconsequential.

[eve.lederman]

Hi! I'm still not understanding why would should eliminate ( C) . It's eliminating an alternate cause, so it's eliminating the second possibility (that a meteoroid hit earth). How is that not helping in strengthening the conclusion?

[maryadkins]

Hi! I'm still not understanding why would should eliminate ( C) . It's eliminating an alternate cause, so it's eliminating the second possibility (that a meteoroid hit earth). How is that not helping in strengthening the conclusion?

It isn't eliminating an alternative cause that's possible in the confines of the question.

The stimulus gives you ONLY TWO WAYS that there can be a moon made from the planet it orbits:

1. From the inside of the planet.
2. From the outside of the planet.

This isn't given to you as the opinion or conclusion you need to support. It's given as a premise. That means you have to view it as 100% true.

(C) thus becomes irrelevant. The only way the argument is lacking—the only assumption it's making—is that the moon wasn't made from the inside of the earth.