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I have a question about a practice test!! Why is the correct answer D? I chose C.

Ir has not been common at Earth’s surface since the very beginning of the planet’s history. Because it usually exists in a metallic state, it was preferentially incorporated in Earth’s core as the planet cooled and consolidated. Ir is found in high concentrations in some meteorites, in which the solar system’s original chemical composition is preserved. Even today, microscopic meteorites continually bombard Earth, falling on both land and sea. By measuring how many of these meteorites fall to Earth over a given period of time, scientists can estimate how long it might have taken to deposit the observed amount of Ir in the boundary clay. These calculations suggest that a period of about one million years would have been required. However, other reliable evidence suggests that the deposition of the boundary clay could not have taken one million years. So the unusually high concentration of Ir seems to require a special explanation.
Paragraph 5 implies that a special explanation of the Ir in the boundary clay is needed because
A. the Ir in microscopic meteorites reaching Earth during the Cretaceous period would have been incorporated into Earth’s core
B. the Ir in the boundary clay was deposited much more than a million years ago
C. the concentration of Ir in the boundary clay is higher than in microscopic meteorites
D. the amount of Ir in the boundary clay is too great to have come from microscopic meteorites during the time the boundary clay was deposited

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Naina B. | Naina, a versatile tutorNaina, a versatile tutor
4.8 4.8 (155 lesson ratings) (155)
Hi Saori,
Answer C is correct in terms of geological and scientific fact. However, the question is about the scientific explanation of unusually high concentration of iron  in the boundary clay.  Statement in   D provides that answer but statement does not. Therefore, although statement C is a correct scientific fact yet it is not the correct answer to question.
Answer A is not correct as paragraph does not talk about cretaceous period.
Answer B is not correct either since paragraph does not include information about measurement of iron so many million years ago.
Hope this helps.


Saori and Naina,

Ir is iridium, not iron. Iron is Fe.  And the statement C is not a correct scientific fact.  The correct answer is D, but not for reasons you might suspect.  You remember, Saori, how I told you about that asteroid impact that was instrumental in killing off the dinosaurs 65 million years ago?  Well the proof that such an impact occurred is based on the unusually high concentration of Ir in a clay layer deposited at that same time! This discovery was made by Luis and Walter Alvarez.
I am surprised, Saori, that the people who wrote this test you're taking are so many decades behind in their scientific assumptions.  Maybe you should question someone about this.
Here is a link to a website which will tell you about the Alvarez discovery:
Thank you so much William, now I recall it. I just assumed Saori typed Ir instead of Iron.
Thank you, Naina and William!!
I read the website, and I though was actually surprised from the article.
And, the hypothesis are changing over time.
Thank you for your help!!