Explanation

@Boram, Happy to help! The question says, "The author implies that if the theory described in the third paragraph is true, which of the following would have been true of iridium in the atmosphere at the end of the Cretaceous period?" Before looking at the answer choices, let's look at the third paragraph and see if we can make a prediction about iridium in the atmosphere. The relevant portion of the third paragraph we need to look at is lines 39 through 47. These lines mention that iridium hexafluoride would be emitted from a diapir eruption and that iridium hexafluoride is a gas that would disperse more uniformly in the atmosphere than the iridium-containing matter thrown out from a meteorite impact. Based on this, we know we're looking for an answer that will say something about there being an even distribution of iridium in the atmosphere at the end of the Cretaceous period. Answer D says, "It would have been more uniformly dispersed than iridium whose source had been the impact of a meteorite on the Earth's surface." This is the implication that the author is making in lines 39 through 47, so this is the correct answer. Answers A, B, C, and E are incorrect because while they're not necessarily contradictory to what the author is saying, none of these answers is something we that the author is implying. With D, however, we have clear textual support for the author's implications of an even distribution of iridium, so that is why it's our answer. Does this make sense? Let us know if you have any more questions!

It seems to me that the question wrongfully has us reference the 3rd paragraph, but actually was referring to the 4th paragraph with lines 39 through 47. The 3rd paragraph begins with "Currently available" while the 4th paragraph begins with "Moreover". The 3rd paragraph also does not mention iridium, which would make finding the correct answer difficult.

You're correct that the theory is discussed in the third paragraph, but we're looking for a consequence of the theory actually being true. This is what takes us to the 4th paragraph to the lines you mentioned. If the volcanic eruption theory were true, the fourth paragraph tells us that the iridium in the atmosphere would have likely been released as iridium hexafluoride, which would disperse more uniformly than iridium from a meteorite impact. That's why D is correct.