请教Magoosh上的一道阅读题？

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In the mid-1970’s, Walter Alvarez, a geologist, was studying Earth’s polarity. It had recently been learned that the orientation of the planet’s magnetic field reverses, so that every so often, in effect, south becomes north and vice versa. Alvarez and some colleagues had found that a certain formation of pinkish limestone in Italy, known as the scaglia rossa, recorded these occasional reversals. The limestone also contained the fossilized remains of millions of tiny sea creatures called foraminifera. Alvarez became interested in a thin layer of clay in the limestone that seemed to have been laid down around the end of the Cretaceous Period. Below the layer, certain species of foraminifera—or forams, for short—were preserved. In the clay layer, there were no forams. Above the layer, the earlier species disappeared and new forams appeared. Having been taught the uniformitarian view, which held that any apparent extinctions throughout geological time resulted from ‘the incompleteness of the fossil record’ rather than an actual extinction, Alvarez was not sure what to make of the lacuna in geological time corresponding to the missing foraminifera, because the change looked very abrupt.

Had Walter Alvarez not asked his father, the Nobel Prize-winning physicist Luis Alvarez, how long the clay had taken to deposit, the younger Alvarez may not have thought to use iridium, an element rarely found on earth but more plentiful in meteorites, to answer this question. Iridium, in the form of microscopic grains of cosmic dust, is constantly raining down on the planet. The Alvarezes reasoned that if the clay layer had taken a significant amount of time to deposit, it would contain detectable levels of iridium. The results were startling: far too much iridium had shown up. The Alvarez hypothesis, as it became known, was that everything—not just the clay layer—could be explained by a single event: a six-mile-wide asteroid had slammed into Earth, killing off not only the forams but also the dinosaurs and all the other organisms that went extinct at the end of the Cretaceous period.

问题：The Alvarezes concluded that a meteorite was responsible for a mass extinction because?
A:dinosaurs were prone to the effects of iridium
B:it could account for the heightened presence of an element
C:cosmic dust in the form of iridium constantly makes its way to the Earth’s surface
D:the scaglia rossa had a sudden gap in traceable iridium
E:the scientific community was unable to provide a more valid hypothesis

答案是B， 我感觉BD都对 ，只不过个人感觉逻辑上B更直接的回到了该问题，而D稍微绕了点弯。
D之所以错，Magoosh给的答案解释说是：(D) is wrong because the scaglia rossa showed a gap in foraminifera, not iridium. 当然我们从文中直接知道forams有一个gap在limestone (limestone即 scaglia rossa)里，即clay layer没有formas，（参考 Below the layer, certain species of foraminifera—or forams, for short—were preserved. In the clay layer, there were no forams. Above the layer, the earlier species disappeared and new forams appeared.），但是我们据此至少还知道limestone是分层的（可以简单的说是above clay, clay and below clay）。
2：从最后一段我们知道clay layer中有很多Iridum（参考The results were startling: far too much iridium had shown up.），因此limestone中将会有一个 iridum的gap，即clay layer有很多iridum, 而其它层则很少。
所以D对而且也解释了问题，即iridum的在limestone中clayer层有很多，说明了彗星撞地球理论。

还请各位热心人士能帮忙解答下啊？