Five cloned pigs，whose organs are much less likely to be rejected(排斥) by a patient，have been born in the U．S. More than 62,000 people in the U．S. alone are waiting to ____ donated hearts，lungs and so on.The number of human donors falls far short of ____.Pig organs are of a(n) ____ size to human organs，and some scientists hope they might be used to help meet the ____.But previous attempts to transplant pig tissue into humans have ____. The five pigs ____ a gene that adds a sugar to the surface of pig cells.The sugar would ____ immune(免疫的) rejection of the tissue.“This advance provides a neartime ____ for overcoming the problem that there is not enough human organs for transplants，”says an expert.“This is the ____ gene for overcoming the ____ stage of rejection.” ____，scientists warn that much more work is necessary ____ organs from copies of the pigs could be transplanted into humans.Human genes will need to be added，to ____ rejection of the organ in the long term.There are also ____ that pig viruses could infect patients. Cloning techniques were ____ to the production of the pigs.Genes can only be knocked out(去除) in a single cell.Cloning of these single cells then allowed the ____ of a whole animal in which the gene was knocked out in every cell.But the PPL researchers have ____ in knocking out only one copy of the gene.The team will now attempt to knock out both copies of the gene. The team will also ____ tests to investigate whether a virus from the pigs could infect human cells.“Although a lot of the work is very ____，we’re still very far off being able to grow an organ，” says Julia，who is working on this project and quite ____ to creating similar knockout pigs with researchers at the University of Missouri.
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小题1:C
小题2:D
小题3:A
小题4:B
小题5:C
小题6:A
小题7:C
小题8:D
小题9:A
小题10:C
小题11:B
小题12:C
小题13:A
小题14:D
小题15:A
小题16:B
小题17:A
小题18:B
小题19:C
小题20:D
小题1: D
解析 由本段内容可知，应是等待接受器官的移植。
小题2: B
解析 器官捐献不能满足人们的需求，demand符合语境。
小题3: C
解析 根据上下文可知，克隆猪的器官可以被移植到人体内，它们的器官应该和人类的器官大小相似。
小题4: A
解析 根据本段第二句“The number of human donors falls far short of ____.”可知，克隆猪的器官是为了弥补人类可移植器官的不足。
小题5: B
解析 上一句提到科学家期望克隆猪的器官能够应用于人类器官移植领域，but表示转折，说明以前类似的试验失败了。
小题6: A
解析 由下文“...a gene that adds a sugar to the surface of pig cells.The sugar would ____ immune(免疫的) rejection of the tissue.”可推测，这一基因会导致不好的结果，又根据“‘This advance provides...’”可知，这五只克隆猪没有这一基因，因此选lack。
小题7: B
解析 根据上下文可知，这种糖分可能会使器官产生免疫排斥，cause符合语境。
小题8: D
解析 根据后面overcoming the problem的提示，可知对人类可移植器官不足的问题提供了一个解决办法。
小题9: C
解析 根据上文“...a gene that adds a sugar to the surface of pig cells.”可知，这一基因对克服器官排斥问题很关键，所以选key。
小题10: D
解析 根据下一段第二句“Human genes will need to be added，to ____ rejection of the organ in the long term.”可知，对这一基因的处理只能解决器官排斥早期的一些问题。
小题11: A
解析 根据下文“...scientists warn that much more work is necessary...”可知，本段内容与上一段意思是转折关系，所以用however。
小题12: B
解析 根据下文“Human genes will need to be added...”可知，在克隆猪的器官可以移植到人体内之前，还有很多工作要做，所以用before。
小题13: A
解析 根据上下文可知，科学家们的这些工作是为了防止器官排斥。
小题14: C
解析 由句意可知，猪身上的病毒可能会感染病人，这是人们所担心的。
小题15: B
解析 这五只猪是克隆动物，显而易见，克隆技术对繁育出这几只猪至关重要，vital符合语境。
小题16: D
解析 基因的去除只有在单个细胞中才能进行，克隆这些单细胞然后再将其培育成一只完整的动物，这样动物身上的每个细胞中的这种基因就都被去除了，creation符合语境。
小题17: B
解析 根据下文“The team will now attempt to knock out both copies of the gene.”可知，PPL researchers已经成功去除这种基因的一个副本。
小题18: A
解析 句意为：这一团队还将进行试验研究克隆猪身上的细菌是否会感染人体。conduct tests进行试验。
小题19: C
解析 although表示让步，根据主句“‘we’re still very far off being able to grow an organ，’”可知，exciting符合语境。
小题20: C
解析 Julia致力于培育这种去除基因的克隆猪，devoted符合语境。