单选题 During its formative years, the inner solar system was a rough-and-tumble place. There were a couple of hundred large objects flying around. Moon-size or bigger, and for millions of years they collided with one another. Out of these impacts grew the terrestrial planets—Mercury, Venus, Earth with its Moon, and Mars—and the asteroids.
Scientists have thought of these collisions as mergers: a smaller object (the impactor) hits a larger one (the target) and sticks to it. But new computer modeling by Erik Asphaug and Craig B. Agnor of the University of California, Santa Cruz, shows that things weren't that simple. "Most of the time, the impactor and the target go off on their merry ways," Dr. Asphaug said. About half the collisions are these hit-and-nm affairs. Now the two researchers and a colleague, Quentin Williams. have done simulations to study the effects of these collisions on the impactors. They are not pretty.
"The impactors suffer all kinds of fates," Dr. Asphaug said. They undergo tremendous shearing and gravitational forces that can cause them to fracture into smaller pieces or melt, causing chemical changes in the material and loss of water or other volatile compounds. Or the crust and cover can be stripped off. leaving just an embryonic iron core.
The researchers, whose findings are published in Nature, discovered that two objects did not even have to collide to create an effect on the smaller one. from the gravitational forces of a near-collision. During the simulations. Dr. Asphaug said, "We'd look and say, 'Gosh, we just got rid of the whole atmosphere of that planetoid: it didn't even hit and it sucked the whole atmosphere off.'"
The researchers suggest that the remains of these beaten-up, fractured and melted objects can be found in the asteroid belt. Dr. Asphaug said that could explain the prevalence of "iron relics" in the belt. Some of these planetoid remnants also eventually hit Earth: that would help explain why certain meteorites lack water and other volatile elements.
The hit-and-run collision model also provides an explanation for Vesta. a large asteroid with an intact crust and cover. How did Vesta keep its cover while so many other objects were losing theirs? Dr. Asphang said it could be that Vesta was always the target, never the impactor, and was thus less affected. "It just had to avoid being the hitter," he said, "until bigger objects left the system./

单选题 The planets were formed as a result of______.

【正确答案】 A

【答案解析】推理判断题。题目是要找出行星形成的起源，根据第一段，在太阳系形成阶段，太阳系各种物体相互碰撞，结果就形成了行星。A“太阳系物体碰撞”，与文章信息吻合；B“小物体与大物体融合”，第二段中指出这种观点是以前的看法，现在人们又发现大多数的碰撞不会产生融合，只是"hit-and-run affairs"：C“碰撞物粘到目标物上”，可参考选项B的解释；D“化学变化”，这不是本文的观点。故选A。

单选题 In the last sentence of the second paragraph, "they" refers to______.

【正确答案】 D

【答案解析】推理判断题。根据上面一句，“Now the two researchers and a colleague, Quentin Williams, have done simulations to study the effects of these collisions on the impactors.”(这两个研究者及其同事Quentin Williams已经进行了模拟来研究碰撞对碰撞物的影响。)接着后面一句是"They are not pretty.”下面一段开头"The impactors suffer all kinds of fates"是进一步的阐释，后面说其产生了种种变化，外壳都有可能被剥去，因此可以推断，上面说的是这些碰撞物不好看。答案为D。

单选题 Certain meteorites lack water and other volatile elements probably because______.

【正确答案】 B

【答案解析】推理判断题。根据题目找出第五段最后一句与此有关：“也许这可以说明为什么某些陨石上没有水和其他择发性元素”。追溯本源，前面讲到这些陨石是那些碰撞物体的残留物。第三、四段也都与此有关，第三段提到“They undergo tremendous shearing and gravitational forces that can cause them to fracture into smaller pieces or melt, causing chemical changes in the material and loss of water or other volatile compounds.”(碰撞物发生了一系列变化，分成小块、融合，发生化学变化致使水和其他挥发性成分丢失。)因此最根本的根源在于此。选项B是正确的。

单选题 According to the hit-and-nm collision model, Vesta keeps its cover probably because______.

单选题 The model developed by Erik Asphaug and Craig B. can explain the following facts EXCEPT______.

【正确答案】 C

【答案解析】事实细节题。本文主要论述了Erik Asphaug和Craig B. Angor关于行星形成的新模型，这个模型和以往模式不同，解释了许多现象，现在看选项中哪些是文中提到可以被解释的。A选项从第五段中“Dr. Asphaug said that could explain the prevalence of 'iron relics' in the belt.”一句，可以看出是能解释的；第五段最后一句就证明了B选项；C“行星是因碰撞而产生的”，关于这点文章第一段有相关论述，但并不是这个模式所解释的；D选项从第四段可以看出，“The researchers, whose findings are published in Nature, discovered that two objects did not even have to collide to create an effect on the smaller one, from the gravitational forces of a near-collision.”两个物体无需碰撞都可以产生影响，这也是模式可以解释的。