#19 - Researchers working in Western Australia have

[actionjackson]

For this Must be True question I had narrowed down my options to both A and D. Keeping A as a contender and ultimately choosing answer choice D, I'm a bit confused as to why A is correct and D incorrect. Answer choice A seemed a bit too much/too extreme based on the information contained in the stimulus. When I was met with answer choice A I thought, "well this could be true but I don't know that it MUST be true that the Earth's crust took no longer than 300 million years to form", I looked at A with uncertainty. D seemed to me as being the answer choice that must be true on the basis of the information provided in the stimulus. Where the author posited that these researchers had discovered the oldest fragments of the Earth's early crust, along with these fragments shedding light on how long it took the Earth's crust to form both seemed to indicate to me that these fragments (microdiamonds) were the first components of the Earth's crust to form. Would D have been correct if it had stated that microdiamonds were among the first components of the Earth's crust to form? Any help is appreciated, I just don't see how A is correct here. Thank you.

[Jonathan Evans]

Hi, ActionJackson,

Good questions. First, let's talk extreme language in Must Be True answer choices. The presence of extreme language in a Must Be True answer choice should indeed raise a red flag for you. When you see it, you should go, "Whoa! Watch out there. My first instinct is to call it a Loser."

However, extreme language does not in and of itself make an answer choice wrong. Consider the following hypothetical:

• All milk cows are black and white. All milk cows are mammals. Farmer Joe has ten milk cows in his barn.

Now here's a valid inference:

• No black and white milk cows are not mammals.

Notice there is extreme language here, but it is warranted and supported by the information in the stimulus. Therefore, when you encounter extreme language it should raise flags, as mentioned above, but you need not (and cannot) get rid of it entirely just on the basis of extreme language. If the extreme language makes the answer choice unsupported, adios! But if the extreme language is supported by adequate evidence in the stimulus, it can be present in a credited response.

In this question, the statement in Answer Choice (A) is actually supported by the stimulus. Given that:

• (1) The discovery of microdiamonds sheds light on how long it took for the Earth's crust to form.
  
  AND
  
  (2) Microdiamonds' age dates to a time 300 million years after the formation of the Earth.

We can infer properly that the Earth's crust must have been in some state of formation 300 million years after the formation of the Earth itself. Consider the logical negation of the statement in Answer Choice (A):

• *The Earth's crust took longer than 300 million years to start to form.*

If this were true, then (1) ("The discovery of microdiamonds sheds light on how long it took for the Earth's crust to form.") could not be true. It would be a nonsensical statement.

Therefore, Answer Choice (A) does have adequate support in the stimulus and is the credited response.

Now Answer Choice (D) does in fact have its own issue with extreme language. We do not have adequate support for the statement that "Microdiamonds were the first components of the Earth’s crust to form."

However, even in consideration of your proposal to change (D) to state that "Microdiamonds were among the first components of the Earth’s crust to form," we still could not make this inference. It could still be possible that many other parts of the crust predated the formation of microdiamonds but these parts of the crust either no longer exist or are yet to be discovered.

Great questions. I hope this helps!

[bk1111]

Hi,

Can someone please elaborate on why A is correct? I am failing to understand how to draw that conclusion based on the stimulus. I ended up picking C, which must be false, but I still don't understand. I have spent a solid 10-15 minutes on this so I am clearly missing something here.

I think I am particularly confused by microdiamonds, which are a part of the earth's crust, being formed 4.2 billion years ago.

Thank you!

[Luke Haqq]

Hi bk1111!

To see why (A) has to be right on this question (a must be true question), let's start by supposing (A) was not true. Suppose, for example, that the earth's crust took 500 million years to form. The stimulus tells us two important facts about microdiamonds: (1) they're parts of Earth's "early crust," (2) they were formed 300 million years after the earth itself was formed. If both of those were true, it couldn't be possible that the crust took 500 million years to "start to form."

Rather, those two facts imply that it has to be the case that "The Earth’s crust took no longer than 300 million years to start to form." (note that talking about how long it took the crust to "start to form" is different from talking about how long it took for the crust to "completely form" or to "finish forming") Answer (A) has to be the case because we have proof (microdiamonds) that the crust had already "started to form" within the first 300 million years (because microdiamonds are part of the "early crust"). Microdiamonds thus demonstrate that certainly the crust had started to form within the first 300 million years, and it's possible the crust started earlier than that--that's why it must be true that starting-to-form took "no longer than 300 million years."

Lastly, given the difference I've mentioned between "starting to form" and "completely forming," this suggests why (C) wouldn't be right for a must-be-true question. We don't know how long it took to for the crust to form (overall), as answer (C) suggests--it could be true that it took billions of years, but we don't know this. Rather, we only know an outer limit of how long it took for the crust to "start to form"--parts of the early crust had started to be present within the first 300 million years (with that outer limit being 4.2 billion years ago).

Hope that helps!

[harvoolio]

I had also selected (C) assuming that the micro-diamonds were the building blocks of the crust and the discovery shed light on on how long it took for the Earth's crust to form by extending the timeline (i.e. early building blocks were present in the Earth's crust).

But I guess even with my erroneous reasoning, (D) basically says the same thing so I could have eliminated (C) too (if I had not realized my erroneous reasoning).

Thanks.

[Daniel Stern]

Harv:

C is not supported by the passage because the only information we are given about the crust is that microdiamonds are the oldest part of it that we've identified, and they are 4.2 billion years old, and that's only 300 million years after the formation of earth itself. The stimulus doesn't tell us enough to infer anything about the total length of time it took the crust to form.

D is not supported because the microdiamonds are the oldest part of the crust we've identified "yet." There might be some older rock that is part of the crust waiting to be identified, so we can't conclude D.

Remember that your answer on a must be true question has to be 100% supported by the stimulus.

Best,
Dan