CAT Critical Reasoning Practice question with Solution 94

Sedimentary rock hardens within the earth's crust as lavers of matter accumulate and the pressure of the layers above converts the layers below into rock. One particular layer of sedimentary rock that contains an unusual amount of the element iridium has been presented as support for a theory that a meteorite collided with the earth some sixty million years ago. Meteorites are rich in iridium compared to the earth's crust, and geologists theorize that a meteorite's collision with the earth raised a huge cloud of iridium-laden dust. The dust, they say, eventually settled to earth where it combined with other matter, and as new layers accumulated above it, it formed a layer of iridium-rich rock.

Which one of the following, if true, would counter the claim that the iridium-rich layer described in the passage is evidence for the meteorite collision theory?

[A]. The huge dust clo8ud described in the passage would have blocked the transmission of sunlight and lowered the earth's temperature.
[B]. A layer of sedimentary rock takes millions of years to harden.
[C]. Layers of sedimentary rock are used to determine the dates of prehistoric events whether or not they contain iridium.
[D]. Sixty million years ago there was a surge in volcanic activity in which the matter spewed from the volcanoes formed huge iridium-rich dust clouds.
[E]. The iridium deposit occurred at about the same time that many animal species became extinct and some scientists have theorized that mass dinosaur extinctions were caused by a meteorite collision.
Answer: D

The geologists figure that the extra iridium in that rock layer had to come from somewhere. Meteorites are rich in iridium; the geologists reason that a meteor could have hit the earth and produced a huge cloud of iridium-rich dust. This dust would settle on the ground, and the ground would eventually become a rock layer containing a lot of iridium. All well and good, but (D) provides an equally good, alternative explanation for the layer of iridium-rich rock. The iridium didn’t have to come from a meteorite; it could have come from the volcanoes.

(A) is irrelevant. Nowhere in the passage is it implied that the scientists’ hypothesis depends on the earth’s temperature not falling.

(B), too, leaves the argument unscathed. The scientists’ hypothesis is that a meteorite hit the earth sixty million years ago, leaving plenty of time, in (B)’s terms, for the dust to settle and a layer of sedimentary rock to form.

(C) The argument only concerns the origin of this one particular layer, not the dating of sedimentary rock in general.

(E) gives another reason for believing that a meteorite hit the earth at about that time. But we want to weaken the meteorite theory, something (E) certainly doesn’t do.

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