首先，通过剑桥9Test1 Passage2<Is Anybody Out There>的分析，我们来确认一下。
READING PASSAGE 2
You should spend about 20 minutes on Questions 14-26, which are based on ReadingPassage 2 on the following pages.
Reading Passage 2 has five paragraphs, A-E.
Choose the correct heading for paragraphs B-E from the list of headings below.
Write the correct number, i-vii, in boxes 14-17 on your answer sheet.
List of Headings
i Seeking the transmission of radio signals from planets
ii Appropriate responses to signals from other civilisations
iii Vast distances to Earth's closest neighbours
iv Assumptions underlying the search for extra-terrestrial intelligence
v Reasons for the search for extra-terrestrial intelligence
vi Knowledge of extra-terrestrial life forms
vii Likelihood of life on other planets
Paragraph A v
14 Paragraph B
15 Paragraph C
16 Paragraph D
17 Paragraph E
IS THERE ANYBODY OUT THERE?
The Search for Extra-terrestrial Intelligence
The question of whether we are alone in theUniverse has haunted humanity for centuries,but we may now stand poised on the brink ofthe answer to that question, as we search forradio signals from other intelligent civilisations.This search, often known by the acronym SETI(search for extra-terrestrial intelligence), is adifficult one. Although groups around the worldhave been searching intermittently for threedecades, it is only now that we have reachedthe level of technology where we can make adetermined attempt to search all nearby starsfor any sign of life.
The primary reason for the search is basic curiosity - the same curiosity about the naturalworld that drives all pure science. We want to know whether we are alone in the Universe.We want to know whether life evolves naturally if given the right conditions, or whether thereis something very special about the Earth to have fostered the variety of life forms thatwe see around us on the planet. The simple detection of a radio signal will be sufficient toanswer this most basic of all questions. In this sense, SETI is another cog in the machineryof pure science which is continually pushing out the horizon of our knowledge. However,there are other reasons for being interested in whether life exists elsewhere. For example,we have had civilisation on Earth for perhaps only a few thousand years, and the threats ofnuclear war and pollution over the last few decades have told us that our survival may betenuous. Will we last another two thousand years or will we wipe ourselves out? Since thelifetime of a planet like ours is several billion years, we can expect that, if other civilizationsdo survive in our galaxy, their ages will range from zero to several billion years. Thus anyother civilisation that we hear from is likely to be far older, on average, than ourselves. Themere existence of such a civilisation will tell us that long-term survival is possible, and givesus some cause for optimism. It is even possible that the older civilisation may pass on thebenefits of their experience in dealing with threats to survival such as nuclear war and globalpollution, and other threats that we haven't yet discovered.
In discussing whether we are alone, most SETI scientists adopt two ground rules. First.UFOs (Unidentified Flying Objects) are generally ignored since most scientists don'tconsider the evidence for them to be strong enough to bear serious consideration(although it is also important to keep an open mind in case any really convincing evidenceemerges in the future). Second, we make a very conservative assumption that we arelooking for a life form that is pretty well like us, since if it differs radically from us we maywell not recognise it as a life form, quite apart from whether we are able to communicatewith it. In other words, the life form we are looking for may well have two green headsand seven fingers, but it will nevertheless resemble us in that it should communicate withits fellows, be interested in the Universe, live on a planet orbiting a star like our Sun. andperhaps most restrictively, have a chemistry, like us, based on carbon and water.
Even when we make these assumptions, our understanding of other life forms is stillseverely limited. We do not even know, for example, how many stars have planets, and wecertainly do not know how likely it is that life will arise naturally, given the right conditions.However, when we look at the 100 billion stars in our galaxy (the Milky Way), and 100billion galaxies in the observable Universe, it seems inconceivable that at least one ofthese planets does not have a life form on it; in fact, the best educated guess we canmake, using the little that we do know about the conditions for carbon-based life, leads usto estimate that perhaps one in 100,000 stars might have a life-bearing planet orbitingit. That means that our nearest neighbours are perhaps 100 light years away, which isalmost next door in astronomical terms.
An alien civilisation could choose many different ways of s