#6 - The largest volcano on Mars rises 27 kilometers above

[altheaD]

Hello,
I understand that d is the right answer, but I'm not quite sure why e is not also correct. based on the explanation in the stimuls, the crust being too thin is the only reason for earth's mountain not being able to be as large as it is on Mars.

Or... am I being presumptuous that the thin crust is the ONLY reason? Is that why e is wrong? I appreciate confirmation of this suspicion... or if there is another reason, what that might be.

Thanks!

[James Finch]

Hi Althea,

Yes, that's exactly it! Good job! We can see this clearly if we conditionalize the relationship given in the stimulus:

Giant Volcano (GV) Thick Crust (TC)

and its contrapositive:

TC GV

We're given:

GVMars

allowing us to infer:

GVMars TCMars

Which is reflected by the correct answer choice, (D).

(E) reflects a Mistaken Reversal of what we would know, as the thick crust is a necessary condition here, not a sufficient one; we only know that when there isn't a thick crust, we can't have a giant volcano, as is the case with Earth:

TCEarth GVEarth

If Earth has a TC, we only know the necessary condition, but that doesn't tell us whether the sufficient condition is true or not, as this isn't a biconditional relationship.

Hope this helps!

[ericj_williams]

Hi Althea,

Yes, that's exactly it! Good job! We can see this clearly if we conditionalize the relationship given in the stimulus:

Giant Volcano (GV) Thick Crust (TC)

and its contrapositive:

TC GV

We're given:

GVMars

allowing us to infer:

GVMars TCMars

Which is reflected by the correct answer choice, (D).

(E) reflects a Mistaken Reversal of what we would know, as the thick crust is a necessary condition here, not a sufficient one; we only know that when there isn't a thick crust, we can't have a giant volcano, as is the case with Earth:

TCEarth GVEarth

If Earth has a TC, we only know the necessary condition, but that doesn't tell us whether the sufficient condition is true or not, as this isn't a biconditional relationship.

Hope this helps!

I don't agree.

I picked D but I don't think it must be true.

What in the stimulus says that it has to be thicker at all.

What would be wrong with saying that they are the same thickness, the difference is entirely gravity?

It says even if they were the same (gravity), which leads me to believe that they are not likely the same. So now we have to deal with a combination of both gravity and thickness explaining the ability to support the volcanoes. Without knowing the relative qualities of each, we really can't conclude anything.

[ericj_williams]

Hi Althea,

Yes, that's exactly it! Good job! We can see this clearly if we conditionalize the relationship given in the stimulus:

Giant Volcano (GV) Thick Crust (TC)

and its contrapositive:

TC GV

We're given:

GVMars

allowing us to infer:

GVMars TCMars

Which is reflected by the correct answer choice, (D).

(E) reflects a Mistaken Reversal of what we would know, as the thick crust is a necessary condition here, not a sufficient one; we only know that when there isn't a thick crust, we can't have a giant volcano, as is the case with Earth:

TCEarth GVEarth

If Earth has a TC, we only know the necessary condition, but that doesn't tell us whether the sufficient condition is true or not, as this isn't a biconditional relationship.

Hope this helps!

I don't agree.

I picked D but I don't think it must be true.

What in the stimulus says that it has to be thicker at all.

What would be wrong with saying that they are the same thickness, the difference is entirely gravity?

It says even if they were the same (gravity), which leads me to believe that they are not likely the same. So now we have to deal with a combination of both gravity and thickness explaining the ability to support the volcanoes. Without knowing the relative qualities of each, we really can't conclude anything.

The problem is that the stimulus says EVEN IF. This implies a possibility, so how can we conclude anything definitely when our premise begins with the something possible.

EVEN IF gravity were the same, Earth would not be thick enough. But we are not told gravity IS the same, so how can we conclude anything about thickness, when thickness is dependent on a possibility. Were saying that something must be true based on a premise dealing in possibility.

[Jeremy Press]

Hi Eric,

I'm a little confused by your last question, and your comments on the phrase "even if." But I have to tell you, I don't think you're going to win this Must Be True battle with this question (answer choice D is solid). The "even if" sentence in this stimulus is giving us a very firm basis on which to build our inference. It's allowing you to imagine a scenario in which Earth's gravity were reduced as low as Mars's (to the same level of Mars or maybe even lower). In that scenario, Earth's crust would not support that Mars volcano. What does that tell us about Mars, where the crust does support such a volcano at that lower level of gravity? The crust must be capable of supporting it, i.e. the crust must be thick enough to support it. Thicker than Earth's, which would not support it at that level of gravity! And there's our inference.

If you could clarify your comment on the "even if" sentence, I might be able to provide a little more guidance. I'm just not sure where you're getting the notion that this implies a "possibility" that can't be used to draw an inference.

I hope this helps!