Problem
If
y
is 45 percent of
x
and
z
is 25 percent of
x
, what is
y
+
z
in terms of
x
?
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LSAT: Reading Comprehension Question #1 |
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Since it was proposed in 1980, the Alvarezes' theory that the mass extinction of plant and animal species at the end of the Cretaceous period sixty-five million years ago resulted from a devastating extraterrestrial impact has won increasing support, although even today there is no consensus for it among scientists. In the Alvarezes' scenario, an asteroid 10 kilometers in diameter struck the earth at high velocity, forming a crater 150 kilometers wide. In addition to the immediate devastation of tidal waves, global fires, and giant storms, impact debris hurled into the atmosphere at high altitude spread around the Earth, preventing sunlight from reaching the ground. With photosynthesis blocked, herbivorous and carnivorous species died as the food chain was snapped at its base.
The Alvarezes' primary evidence is a superabundance of iridium in the "Cretaceous/Tertiary boundary" (KT boundary), a thin rock stratum dividing Cretaceous rocks from those of the later Tertiary period. Iridium, relatively rare in the Earth's crust, comes mainly from the slow fall of interplanetary debris; in some KT boundary strata, iridium is 10-100 times as abundant as normal, suggesting a rapid, massive deposition. Coincident with the boundary, whole species of pollens and unicellular animals vanished from the fossil record, strongly supporting the idea of a catastrophic event. Later studies have shown that some KT boundary samples also contain osmium isotopes typical of meteorites, basalt sphericles that may have melted on impact and rapidly cooled in the atmosphere, and quartz grains deformed in a manner typical of high velocity impacts.
Initially, paleontologists dismissed the theory, arguing that fossils of large animals such as dinosaurs showed a gradual extinction lasting millions of years. But recent intensive exploration in the Hell Creek formation of North Dakota and Montana, aimed at collecting all available dinosaur remnants rather than selectively searching for rare or well-preserved fossils, has shown an abundance of dinosaurs right up to the KT boundary. As a result, opposition to catastrophic mass extinction has substantially weakened among paleontologists.
Given the lack of a known impact crater of the necessary age and size, and the fact that the theory requires the extinctions to have occurred in an extremely short time, some scientists have proposed alternative catastrophe scenarios. Courtillot and others have argued that massive volcanic eruptions, lasting hundreds of thousands of years, pumped enough debris into the atmosphere to cause the darkness and chemical changes that devastated life on the planet. Courtillot's evidence includes huge volcanic flows in India that coincide with the KT boundary, and analyses of KT boundary rocks that seem to show that the excess iridium was laid down over 10-100,000 years, too long for the impact hypothesis.
Walter Alvarez and Frank Asaro reply that the shock wave caused by an impact could have melted mantle rocks, triggering the volcanic activity. They concede, though, that the exact mechanism is unclear. Meanwhile, drillings at a 150-kilometer-wide circular geologic formation in Yucatan, found in 1978 but not carefully examined until 1990, have shown a composition consistent with extraterrestrial impact. However, there is still no conclusive evidence that the Yucatan formation is the long-sought impact site.
Question:
Which of the following, if true, would most weaken the theory that the Cretaceous extinctions were caused by the impact of an asteroid?
Choices:
Topic and Scope of Passage:
Mass extinction at the end of the Cretaceous era; specifically, a comparison of two competing theories that seek to explain mass extinction at the end of the Cretaceous era.
Paragraph Structure:
The first sentence of Paragraph 1 does two things: (1) it introduces the Alvarezes' "asteroid impact" theory of mass extinction and, more importantly, (2) the Keyword "although" signals that somewhere in the passage an alternative theory will be discussed. The remainder of Paragraph 1 goes on to outline the asteroid impact theory in more detail.
Paragraph 2, on the other hand, discusses the evidence for the theory: the iridium layer and other particles found at the KT boundary.
The Keyword "Initially" unlocks Paragraph 3. At first, paleontologists discounted this theory, but have since become less resistant because of fossil evidence found at Hell Creek.
Paragraph 4 describes Courtillot's alternative "volcanic" theory, which is also based on the notion of a catastrophe, but an earthly, rather than extraterrestrial, catastrophe.
Paragraph 5 implies that there isn't yet enough evidence to conclude which theory is more credible.
The Question:
Paragraph 4 says that Courtillot's volcanic theory surmises that the iridium layer was generated over a 10,000-100,000 year time frame. If the iridium layer was actually laid down over a 10,000 year period, then Courtillot's alternative theory would be strengthened at the expense of the Alvarezes' asteroid impact theory.
(B) and (E) distort the passage. The asteroid impact theory attempts to explain why sudden massive extinctions of plant and animal species took place at the end of the Cretaceous era.
(C): Paragraph 5 indicates that the asteroid impact theory can account for "extensive volcanic activity" at the time of the extinctions, albeit imperfectly.
(D): The fact that conclusive evidence like an impact crater has yet to be found means that the theory cannot yet be accepted as valid; but that's a far cry from saying that the lack of compelling evidence weakens the theory.
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