- Meteorites are rich in iridium in comparison to the Earth's crust
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One layer of sedimentary rock contains an unusual amount of iridium
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A meteorite caused this iridium (meteorite collision theory)
The main key to this question is understanding the conclusion. "It was a meteorite!" the conclusion says. How can we weaken this? Pretty easily actually. All we have to do is show that it was actually something else that caused this "huge cloud of iridium-laden dust" to form the layer of iridium-rich rock.
- (A) This just talks more about the huge dust cloud. We don't want to know what effect the dust cloud had; we want to know what caused the dust cloud to begin with.
(B) But does this mean that it was/wasn't the meteorite that caused the iridium in the dust cloud? We don't know! All we know is that rock takes millions of years to harden. Okay....
(C) This functions much like (A). We don't care about any effects of the rock. AKA, we don't care how sedimentary rock is used. We care about what caused the iridium in the sedimentary rock.
(D) This looks great! (D) basically says, "nope. It wasn't the meteorite that caused the iridium. It was actually the matter spewed from the volcanoes." In other words, this provides an alternate cause and that is exactly what we wanted to do!
(E) This actually strengthens the conclusion! It does so by giving more credence to the idea that it was a meteorite that caused the iridium dust. Why? Well it says that the iridium formed at about the same time as mass dinosaur extinctions. So what? This is important because many people believe that the dinosaurs were extinct because of, you guessed it...a meteorite! While it doesn't conclusively prove that a meteorite caused the iridium in the rock. It shows that it seems to be correlated with another event that involved a meteorite. Hmm.... Either way, this definitely does not weaken the argument.