在职申硕英语考试

【单选题】For more than thirty years after astronauts first set foot on the Moon, scientists have been unable to unravel the mystery of where theEarth’s only satellite came from.But now there is direct evidence that the Moon was born after a giant collision between the youngEarth and another planet.
Previous studies of rocks from theEarth and the Moon have been unable to distinguish between the two, suggesting that they formed from the same material.But this still left room for a number of theories explaining how—for example, that the Moon andEarth formed from the same material at the same time. It was even suggested that the earlyEarth spun so fast it formed a bulge that eventually broke off to form the Moon.
Franck Poitrasson, and his colleagues at the Swiss Federal Institute of Technology have compared Moon rocks with rocks fromEarth and discovered a surprising difference. They analysed the weight of the elements present in the rock using a highly accurate form of mass spectroscopy(光谱研究) that involves vaporising a sample by passing it through an argon (氩) flame.Although they appeared very similar in most respects, the Moon rocks had a higher ratio of iron-57 to iron-54 isotopes(同位素)than theEarth rocks. "The only way we could explain this difference is that the Moon and theEarth were partly vaporised during their formation," says Poitrasson.
Only the popular "giant planetary impact" theory could generate the temperatures of more than 1700℃ needed to vaporise iron. In this scenario, a Mars-sized planet known as Theia crashed intoEarth 50 million years after the birth of the Solar System. This catastrophic collision would have released 100 million times more energy than the impact believed to have wiped out the dinosaurs—enough to melt and vaporise a large portion of theEarth and completely destroy TheiA、The debris from the collision would have been thrown into orbit around theEarth and eventually coalesced to form the Moon.
When iron is vaporised, the lighter isotopes burn off first.And since the ejected debris that became the Moon would have been more thoroughly vaporised, it would have lost a greater proportion of its lighter iron isotopes thanEarth diD、This would explain the different ratios that Poitrasson has founD、
The main theme of the passage is ______.A．the differences between theEarth and the Moon
B．how man manages to set foot onto the Moon
C．the collision between theEarth and the Moon
D．the origin of the Moon

网考网参考答案：D
网考网解析：
本题是主旨大意题。综合本文各段的主题句即可知本文主要是在阐述月球的起源。 document.getElementById("warp").style.display="none"; document.getElementById("content").style.display="block"; 查看试题解析出处>>