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!

So the third paragraph would be lines 35 to 60? In my book, it seems lines 19 to 34 would be the third paragraph. Lines 1, 10, and 19 have indents so I assumed that those were the beginning of paragraphs one, two and three.

@Boram, I apologize; I accidentally wrote "third" instead of "fourth" in my description above. You're right—the third paragraph is lines 19 through 34 as is in your book. The theory described in lines 19 through 34 is the volcanic eruption theory. It's important to note that with this question, we shouldn't get too carried away with focusing on whether the text is in the third paragraph or not. The test writers put "if the theory described in the third paragraph is true" as a convoluted way of saying "if the volcanic eruption theory were true." And, if this theory were true, we'd expect a uniform dispersion of iridium as lines 39 through 47 state. Sorry about the confusion. I'll try editing my original response so that it says "fourth" instead of "third" where it should. Does this make sense? Let us know if you have any other questions!