根据短文内容，回答下面试题。
　　We all know that DNA has the ability to identify individuals but, because it is inherited, there are also regions of the DNA strand which can relate an individual to his or her family ( immediate and extended), tribal group and even an entire population. Molecular Genealogy (宗谱学) can use this unique identification provided by the genetic markers to link people together into family trees.
　　Pedigrees(家谱) based on such genetic markers can mean a breakthrough for family trees where information is incomplete or missing due to adoption, illegitimacy or lack of records. There are many communities and populations which have lost precious records due to tragic events such as the fire in the Irish courts during Civil War in 1921 or American slaves for whom many records were never kept in the first place.
　　The main objective of the Molecular Genealogy Research Group is to build a database containing over 100,000 DNA samples from individuals all over the word. These individuals will have provided a pedigree chart of at least four generations and a small blood sample. Once the database has enough samples to represent the world genetic make-up, it will eventually help in solving may issues regarding genealogies that could not be done by relying only on traditional written records.
　　Theoretically, any individual will someday be able to trace his or her family origins through this database.
　　In the meantime, as the database is being created, molecular genealogy can already verify possible or suspected relationships between individuals. "For example, if two men sharing the same last name believe that they are related, but no written record proves this relationship, we can verify this possibility by collecting a sample of DNA from both and looking for common markers( in this case we can look primarily at the Y chromosome(染色体) ," explains Ugo Perego, a member of the BYU Molecular Genealogy research team.
　　第21题　People in a large area may possess the same DNA thread because__________.
　　A.DNA is characteristic of a region
　　B.they are beyond doubt of common ancestry
　　C.DNA strand has the ability to identify individuals
　　D.their unique identification can be provided via DNA
　　答案：B
　　第22题　The possible research of family trees is based on the fact that__________.
　　A.genetics has achieved a breakthrough
　　B.genetic information contained in DNA can be revealed now
　　C.each individual carries a unique record of who he is and how he is related to others
　　D.we can use DNA to prove how distant an individual is to a family, a group or a population
　　答案：C
　　第23题　The Molecular Genealogy Research Group is building a database for the purpose of__________.
　　A.offering assistance in working out genealogy-related problems
　　B.solving many issues without relying on traditional written records
　　C.providing a pedigree chart of at least four generations in the world
　　D.confirming the assumption that all individuals are of the same origin
　　答案：A
　　第24题　If two men suspected for some reason they have a common ancestor,__________.
　　A.we can decide according to their family tree
　　B.we can find the truth from their genetic markers
　　C.we can compare the differences in their Y chromosome
　　D.we can look for written records to prove their relationship
　　答案：B
　　第25题　Which of the following CANNOT be inferred from the passage?
　　A.We are a walking, living, breathing record of our ancestors.
　　B.Many American slaves did not know who their ancestors were.
　　C.An adopted child generally lacks enough information to prove his identity.
　　D.Molecular Genealogy can be used to prove a relationship between individuals.
　　答案：C
　　根据短文内容，回答下面试题。
　　There was one thought that air pollution affected only the area immediately around large cities with factories and heavy automobile traffi　　C.At present，we realize that although these are the areas with the worst air pollution，the problem is literally worldwide.On several occasions over the past decade，a heavy cloud of air ponution has covered the east of the United States and brought health warnings in rural areas away from any major concentration of manufacturing and automobile traffi　　C.
　　In fact，the very climate of the entire earth may be infected by air pohufion.Some scienfists consider that the increasing concentration of carbon dioxide in the air resulting from the burning of fossil fuels (coal and oil) is creating a "greenhouse effect"--conserving heat reflected from the earth and raising the world' s average temperature. If this view is correct and the world' s temperature is raised only a few degrees, much of the polar ice cap will melt and cities such as New York, Boston, Miami, and New Orleans will be in water.
　　Another view, less widely held, is that increasing particular matter in the atmosphere is bloc-king sunlight and lowering the earth' s temperature--a result that would be equally disastrous. A drop of just a few degrees could create something close to a new ice age, and would make agriculture difficult or impossible in many of our top fanning areas. Today we do not know for sure that either of these conditions will happen ( though one recent government report drafted by experts in the field concluded that the greenhouse effect is very possible). Perhaps, if we are lucky enough, the two tendencies will offset each other and the world' s temperature will stay about the same as it is now.
　　第26题　As pointed out at the beginning of the passage, people used to think that air pollution__________.
　　A.caused widespread damage in the countryside
　　B.affected the entire eastern half of the United States
　　C.had damaging effect on health
　　D.existed merely in urban and industries areas
　　答案：D
　　第27题　As to the greenhouse effect, the author__________.
　　A.shares the same view with the scientists
　　B.is uncertain of its occurrence
　　C.rejects it as being ungrounded
　　D.thinks that it will destroy the world soon
　　答案：B
　　第28题　The word "offset" in the second paragraph could be replaced by__________.
　　A.slip into　　B.make up for　　C.set up　　D.catch up with
　　答案：B
　　第29题　It can be concluded that__________.
　　A.raising the world' s temperature only a few degrees would not do much harm to life on earth
　　B.lowering the world' s temperature merely a few degrees would lead many major farming are-as to disaster
　　C.almost no temperature variations have occurred over the past decade
　　D.the world' s temperature will remain constant in the years to come
　　答案：B
　　第30题　This passage is primarily about__________.
　　A.the greenhouse effect　　B.the burning of fossil fuels
　　C.the potential effect of air pollution　　D.the likelihood of a new ice age
　　答案：C

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