填空题 Directions:
In the following article, some sentences have been removed. For Questions 41-45, choose the most suitable one from the list A-G to fit into each of the numbered blank. There are two extra choices, which do not fit in any of the gaps. Mark your answers on ANSWER SHEET 1.
What killed the dinosaurs? Until recently, no one really knew the answer to that question. That was until a huge crater was discovered under the sea off Mexico's Yucatan Peninsula. Studies revealed that the hole had been made by an asteroid which struck the earth 65 million years ago, and caused the extinction of the dinosaurs as well as two-thirds of all species.
41. ______
For the first time, humans were able to witness exactly what happens when a celestial body collides with a planet. As the comet punched holes bigger than the earth into Jupiter's atmosphere, and threw up clouds of debris thousands of miles high, it quickly became clear that survival was no longer entirely a question of being the "fittest".
42. ______
But, to most people, the risk remains academic. With all the dangers humans face—sickness, accidents, etc.—it is understandable that people don't take seriously the risk posed by something :hat hasn't happened for 65 million years and may not happen for another 65 million years.
43. ______
Even the worst tornadoes, floods and earthquakes affect only a very small percentage of the earth's surface and population. But the effects of an impact caused by a celestial body of just 10 kilometers in diameter would make humans extinct, along with most of the world's other animals and plants.
The danger comes from asteroids and comets which cross the earth's orbit. Asteroids pose a greater danger because they are more numerous. Most originate in the Asteroid Belt—a vast group of asteroids which orbit the sun between Mars and Jupiter. These asteroids are normally in stable orbits but collisions between them, and the gravitational effects of Jupiter, can change their orbits, either sending them out of the solar system or towards tile sun.
44. ______
They can still do some damage, however, particularly if they are composed of iron. About 50,000 years ago, a metallic asteroid with a diameter of about 30 meters smashed into what is now the American state of Arizona. It left a crater 1.2 kilometers wide.
More recently, in 1908, a rocky asteroid about 60 meters in diameter exploded as it entered the earth's atmosphere above the Tunguska Valley in Siberia.
45. ______
But, it is those asteroids with diameters of one kilometre or more which pose the greatest threat. It is estimated that between 1,000 and 2,000 of these come closer to the earth than Mars, our nearest planetary neighbor. If a one-kilometer asteroid were to hit the earth, the consequences would be devastating. These bodies typically travel at speeds of about 20 kilometers per second. At that speed, one hitting the earth would release energies millions of times greater than those released by the atomic bomb at Hiroshima in 1945. Such an impact would not be enough to make humans extinct, but it would change forever life as we know it. An object of this size is estimated to collide with the earth every 300,000 years.

[A] Once the rain of debris subsided, the sky would turn black. Vast clouds of dust and soot would block out sunlight, stopping photosynthesis and causing acid rain. Then the earth would begin to cool, and a mini ice age would follow. The age of mammals would be over.
[B] Fragments crashed to the ground and although they did not leave a crater, they destroyed an area of forest 50 kilometers across. If this happened above a city, buildings and people would be completely destroyed.
[C] If one were to hit earth now, it would release energies of about 100 million megatons—five billion times greater than the Hiroshima bomb.
[D] A new factor had been introduced into evolution: the ability to survive a collision between the earth and an asteroid or comet.
[E] But, many scientists believe that collisions between the earth and celestial bodies cannot be regarded as "just another risk". The main reason for this is that no other catastrophe—except perhaps a nuclear war—has the potential to destroy human civilization completely.
[F] But just how one impact managed to cause such destruction was not widely understood until July 1994, when the comet Shoemaker-Levy 9 smashed into the planet Jupiter.
[G] Those less than 100 meters in diameter are not usually regarded as a threat because most are destroyed by heat as they enter the earth's atmosphere and so never reach the ground.