Questions 46 and 50 are based on the following passage.-|||-For the past 50 years,scientists have scoured(彻底地搜寻)the skies for radio signals from beyond our-|||-planet , hoping for some sign of extraterrestrial(地球外的)life.But one physicist says there`s no reason alien life-|||-couldn`t already be lurking among us-or maybe even in us.-|||-Paul Davies,an award-wining Arizona Stat University physicist,know for his popular science writing-|||-said that life may have developed on Earth not once but several times.-|||-Davies said the variant life forms-most likely tiny microbes(微生物)-could still be hanging around"right-|||-under out noses -or even in our noses."-|||-"How do we know all life on earth descended from a single origin ?" he told a conference at London`s-|||-prestigious Royal Society "We`ve just scratched the surface of the microbial world."-|||-The idea that alien micro-organisms cou be hiding out here on earth has been discussed for a while-|||-according to Jill Tarter,the director of the U.S.SETI project,which listens for signals from civilizations based-|||-around distant stars.-|||-She said several of the scientists involved in the project were interested in pursuing the notion,which Davies-|||-carlier laid out in a 2007 article published in scientific American in which he asked:"Are alicns among us?"-|||-So far,there`s no answer.And ever finding one would be fraught with(充满......的)difficulti Davies-|||-himself acknowledged.-|||-Unusual organisms abound-including chemical-cating which hide out deep in the ocean and organisms-|||-that thrive in boiling-hot springs-but t doesn`t mean they`re different life forms entirely.-|||-"How weird do they have to be suggest a second origin as opposed to just an obscure branch of the family-|||-tree??"he said D.Davies suggested that the only way to prove an organism wasn`t "life as we know it " was if it-|||-were built using exotic elements which no other form of life had.-|||-Such organisms have yet to be found .Davies also noted that less that I percent of all the world`s bacteria had-|||-been comprehensively stud plenty of time to find unusual organisms.-|||-46.What can we learn from one physicist`s words in Paragraph 1 ?-|||-A)It is unreasonable to discuss the issue of alien life.-|||-B)Life from outer space is likely to be on earth already-|||-C)It`s possible that cxtraterrestrial lif is competing with us secretly-|||-D)Alien life has probably put some extraterrestrial ele in our body-|||-47.What`s the probable meaning of "We`ve just scratched the surface of the microbial world"?-|||-A)We are only living on the surface of the microbial world.-|||-B)The microbial world is too complicated to be studied intensively-|||-C)We haven`t obtained in-depth knowled of the microbial world yet.-|||-D)We can touch such lives as tiny microbes in the microbial world.-|||-48.We can know that the U.S.SETI project __ .-|||-A)is in charge of tracking the development of possible extraterrestrial-|||-B)takes charge of taking over signals about civilizatio from distant stars-|||-C)takers studying messages from planets beyond the earth as its responsibility-|||-D)sees to monitoring signals from possible alien culture to estimate its existence-|||-49.What method did Davies propose to prove the life form of an organism?-|||-A)To speculate if it constitutes an unusual branch of the biological system-|||-B)To figure out whether it has experienced a second time evolvement or not.-|||-C)To see whether it has foreign component unposse by any other life form-|||-D)To make sure whether it is just a common life form or an especially weird one.