I found choice B quite tempting. Although I ultimately chose D, I did not have a very clear reason for eliminating B during my test. Therefore, I'd like to give it a try explaining my thinking. Any comments are really appreciated.
The question asks the purpose of the last sentence of this passage, in particular the word "remarkable correlation". By looking at the structure/roadmap of paragraph 3 starting at line 41, it states that the following evidence supports ocean floor spreading theory and the last sentence is the third evidence of these supports. Therefore, choice D is the answer.
Regarding B, it was tempting at the first look. According to my understanding, the role of last sentence is not an explanation for global mid-ocean ridge. Instead, the last sentence is used to support the ocean floor spreading theory which in turns explains the existence of global mid-ocean ridge (paragraph 2). So the last sentence might be indirectly related to global mid-ocean ridge but the role is not the same as what's stated in choice B.
Regarding other choices,
(A) "Ocean floor spreading occurs at an extremely slow rate" is part of the support evidence that is assumed to make this correlation remarkable. In addition, the correlation and slow spreading rate are different things and cannot indicate each other. Therefore, (A) is wrong.
(C) is out of scope. This passage does not discuss the strength of earth's magnetic field. It only talks about the variations and reversals.
(E) is out of scope too. The passage does not mention whether the earth's magnetic reversals have occurred at regular intervals.
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- ohthatpatrick
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Atticus Finch
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Re: Q17
Great explanation!
Look at these RC question stems:
the author said ____ in order to
the author's mention of ____ serves to
the author brought up ____ primarily to
What do they all have in common? Infinitives. (what a stupid answer)
If I say "Mary smacked the table with a newspaper"
vs.
"Mary smacked the table with a newspaper to scare away a fly"
the infinitive "to scare" tells us THE PURPOSE of her action.
Similarly, any RC question stem worded like those above is asking WHY an author said something. The correct answer to these questions is almost never a paraphrase of the ACTUAL sentence/detail/quote being asked about. That's the WHAT, not the WHY.
The WHY is normally found in some broader claim immediately before or after the WHAT. Or, the WHY is found in the overall point of that paragraph.
As you surmised, the key to getting Q17 correct is to focus on the purpose of the paragraph, as indicated by its topic sentence: "this theory was supported by several lines of evidence."
First - rocks near the crest are youngest
Second - Youngest rocks have normal polarity
Finally - we'd already determined the timeline of magnetic reversals by studying rocks on land ... turns out that these stripes of rock in the ocean match that timeline.
So the 'remarkable correlation' refers to the third LINE OF EVIDENCE SUPPORTING the theory.
(D) is the best paraphrase of that.
I'll slightly tweak some of your answer choice explanations:
(A) if we accept that the stripes correlate with magnetic reversals, then we're basically learning that the ocean floor spreads a few centimeters per year. I'm not sure whether we'd consider that "an extremely slow rate", but it seems pretty gradual. However, the author isn't fascinated by the 'remarkable' correlation because it gives us a cm/year rate for ocean floor spreading. The author thinks the correlation is remarkable because it helps confirm the ocean floor spreading theory. Trap answers to these WHY questions can sometimes a true statement about WHAT we know, but not a correct answer to WHY the author wants us to know it.
(B) The correlation does not EXPLAIN why the mid-ocean ridge exists. It supports the theory that the ocean floor spreads, and ocean floor spreading is thought to cause the mid-ocean ridge. This one is pretty close, but "explains" is too extreme.
(C) the previous poster nailed this --- fake comparison we never talked about concerning strength of field before vs. now
(E) just because we know the ages of the intervals doesn't mean that they occurred at very regular intervals. Also, the correlation between the stripes and the intervals could not be revealing that the intervals were regular. Remember the order of our knowledge here:
1st - We determined the intervals of magnetic reversals (using rocks on land)
2nd - We assumed a constant rate of spreading
Last - We saw that the stripes correlate with those intervals
Since we already knew the intervals of reversals prior to finding the 'remarkable correlation', the latter could not cause us to discover the former.
Hope this helps.
Regarding other choices,
(A) "Ocean floor spreading occurs at an extremely slow rate" is part of the support evidence that is assumed to make this correlation remarkable. In addition, the correlation and slow spreading rate are different things and cannot indicate each other. Therefore, (A) is wrong.
(C) is out of scope. This passage does not discuss the strength of earth's magnetic field. It only talks about the variations and reversals.
(E) is out of scope too. The passage does not mention whether the earth's magnetic reversals have occurred at regular intervals.
Look at these RC question stems:
the author said ____ in order to
the author's mention of ____ serves to
the author brought up ____ primarily to
What do they all have in common? Infinitives. (what a stupid answer)
If I say "Mary smacked the table with a newspaper"
vs.
"Mary smacked the table with a newspaper to scare away a fly"
the infinitive "to scare" tells us THE PURPOSE of her action.
Similarly, any RC question stem worded like those above is asking WHY an author said something. The correct answer to these questions is almost never a paraphrase of the ACTUAL sentence/detail/quote being asked about. That's the WHAT, not the WHY.
The WHY is normally found in some broader claim immediately before or after the WHAT. Or, the WHY is found in the overall point of that paragraph.
As you surmised, the key to getting Q17 correct is to focus on the purpose of the paragraph, as indicated by its topic sentence: "this theory was supported by several lines of evidence."
First - rocks near the crest are youngest
Second - Youngest rocks have normal polarity
Finally - we'd already determined the timeline of magnetic reversals by studying rocks on land ... turns out that these stripes of rock in the ocean match that timeline.
So the 'remarkable correlation' refers to the third LINE OF EVIDENCE SUPPORTING the theory.
(D) is the best paraphrase of that.
I'll slightly tweak some of your answer choice explanations:
(A) if we accept that the stripes correlate with magnetic reversals, then we're basically learning that the ocean floor spreads a few centimeters per year. I'm not sure whether we'd consider that "an extremely slow rate", but it seems pretty gradual. However, the author isn't fascinated by the 'remarkable' correlation because it gives us a cm/year rate for ocean floor spreading. The author thinks the correlation is remarkable because it helps confirm the ocean floor spreading theory. Trap answers to these WHY questions can sometimes a true statement about WHAT we know, but not a correct answer to WHY the author wants us to know it.
(B) The correlation does not EXPLAIN why the mid-ocean ridge exists. It supports the theory that the ocean floor spreads, and ocean floor spreading is thought to cause the mid-ocean ridge. This one is pretty close, but "explains" is too extreme.
(C) the previous poster nailed this --- fake comparison we never talked about concerning strength of field before vs. now
(E) just because we know the ages of the intervals doesn't mean that they occurred at very regular intervals. Also, the correlation between the stripes and the intervals could not be revealing that the intervals were regular. Remember the order of our knowledge here:
1st - We determined the intervals of magnetic reversals (using rocks on land)
2nd - We assumed a constant rate of spreading
Last - We saw that the stripes correlate with those intervals
Since we already knew the intervals of reversals prior to finding the 'remarkable correlation', the latter could not cause us to discover the former.
Hope this helps.
Regarding other choices,
(A) "Ocean floor spreading occurs at an extremely slow rate" is part of the support evidence that is assumed to make this correlation remarkable. In addition, the correlation and slow spreading rate are different things and cannot indicate each other. Therefore, (A) is wrong.
(C) is out of scope. This passage does not discuss the strength of earth's magnetic field. It only talks about the variations and reversals.
(E) is out of scope too. The passage does not mention whether the earth's magnetic reversals have occurred at regular intervals.
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