求助一题阅读！！！不好理解啊

Paragraph 3: The body that impacted Earth at the end of the Cretaceous period was a meteorite with a mass of more than a trillion tons and a diameter of at least 10 kilometers. Scientists first identified this impact in 1980 from the worldwide layer of sediment deposited from the dust cloud that enveloped the planet after the impact. This sediment layer is enriched in the rare metal iridium and other elements that are relatively abundant in a meteorite but very rare in the crust of Earth. Even diluted by the terrestrial material excavated from the crater, this component of meteorites is easily identified. By 1990 geologists had located the impact site itself in the Yucat  region of Mexico. The crater, now deeply buried in sediment, was originally about 200 kilometers in diameter.

3. Which of the following can be inferred from paragraph 3 about the location of the meteorite impact in Mexico?

○ The location of the impact site in Mexico was kept secret by geologists from 1980 to 1990.

○It was a well-known fact that the impact had occurred in the Yucat  region.

○Geologists knew that there had been an impact before they knew where it had occurred.

○The Yucat  region was chosen by geologists as the most probable impact site because of its climate.

答案是3，该怎么理解呢，求大虾帮忙！！

还有一题

Paragraph 7: Earth is a target in a cosmic shooting gallery, subject to random violent events that were unsuspected a few decades ago. In 1991 the United States Congress asked NASA to investigate the hazard posed today by large impacts on Earth. The group conducting the study concluded from a detailed analysis that impacts from meteorites can indeed be hazardous. Although there is always some risk that a large impact could occur, careful study shows that this risk is quite small.

11. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

○Until recently, nobody realized that Earth is exposed to unpredictable violent impacts from space.

○In the last few decades, the risk of a random violent impact from space has increased.

○Since most violent events on Earth occur randomly, nobody can predict when or where they will happen.

○A few decades ago, Earth became the target of random violent events originating in outer space.

这题该如何选呢？？

 Paragraph 2: The source had long been known but not exploited. Early in the century, a  pump had come into use in which expanding steam raised a piston in a cylinder, and atmospheric pressure brought it down again when the steam condensed  inside the cylinder to form a vacuum．This “atmospheric engine,”
            invented by Thomas Savery and vastly improved by his partner．Thomas Newcomen,  embodied revolutionary principles, but It was so slow and wasteful of fuel that it could not be employed outside the coal mines for which it had been designed. In the 1760s, James Watt perfected a separate condenser for the steam, so that the cylinder did not have to be cooled at every stroke；then he devised a way to make the piston turn a wheel and thus convert reciprocating (back and forth) motion into rotary motion. He thereby transformed an inefficient pump of limited use into a steam engine of a thousand uses. The final step came when steam was introduced into the cylinder to drive the piston backward as well as forward thereby increasing the speed of the engine and cutting its fuel consumption.

5. According to paragraph 2，the atmospheric engine was slow because

○it had been designed to be used in coal mines

○the cylinder had to cool between each stroke

○it made use of expanding steam to raise the piston in its cylinder

○it could be operated only when a large supply of fuel was available