4．People have been wondering why elephants do not develop cancer even though they have lifespans（壽命）that are similar to humans，living for around 50 to 70 years．
Now scientists believe they know why．A team at the University of Chicago，US has found that elephants carry a large number of genes that stop tumors（腫瘤）developing．To be precise，they found 20 copies of an anti-tumor gene called TP53 in elephants．Most other species，including humans，only carry one copy．
According to the research，the extra copies of the gene improved the animal's sensitivity to DNA damage，which lets the cells quickly kill themselves when damaged before they can go on to form deadly tumors．
"An increased risk of developing cancer has stood in the way of the evolution of large body sizes in many animals，"the study author Dr Vincent Lynch told The Guardian．If every living cell has the same chance of becoming cancerous（癌變的），large creatures with a long lifespan like whales and elephants should have a greater risk of developing cancer than humans and mice．But across species，the risk of cancer does not show a connection with body mass．
This phenomenon was found by Oxford University scientist Richard Peto in the 1970s and later named"Peto's paradox（悖論）"．Biologists believe it results from larger animals using protection that many smaller animals do not．In the elephant's case，the making of TP53 is nature's way of keeping this species alive．
The study also found that when the same genes were brought to life in mice，they had the same cancer resistance as elephants．This means researchers could use the discovery to develop new treatments that can help stop cancers spreading or even developing in the first place．
"Nature has already figured out how to prevent cancer，"said Joshua Schiffman，a biologist at the School of Medicine，University of Utah，US．"It's up to us to learn how different animals deal with the problem so that we can use those strategies to prevent cancer in people．"
32．Why are elephants unlikely to develop cancer？D
A．They have a rather large body size．
B．Their genes suffer no DNA damage．
C．Certain genes in their body kill existing tumors．
D．They carry many genes to prevent tumors developing．
33．According to the passage，what has been a risk in the evolution of large animals？A
A．Deadly tumors．
B．The huge body mass．
C．Cells killing themselves．
D．Sensitivity to DNA damage．
34．What does the underlined part"This phenomenon"in Paragraph 5 probably refer to？B
A．Larger animals have protection from TP53．
B．The risk of cancer is not related to body size．
C．Larger animals suffer the same risk of cancer as smaller ones．
D．The larger animals are，the bigger risk of cancer they will have．
35．What can be concluded from the passage？C
A．Depending on nature is surely enough to fight against cancer．
B．The TP53 genes have proved useful in stopping cancer in people．
C．Humans are expected to prevent cancer with the making of TP53．
D．The extra copies of elephants'genes are used to cure cancer patients．
分析 本文主要介紹了在美國芝加哥大學的一個團隊發(fā)現(xiàn)大象攜帶大量阻止腫瘤發(fā)展的基因．在動物的進化中腫瘤是極大威脅．而物種患癌癥的幾率和它們的體型沒有關(guān)系．在大象的案例中，TP53的制作是讓這個物種存活的自然方法．因此人們有望通過合成TP53來預防癌癥．
解答 32．D推理判斷題．根據(jù)第二段A team at the University of Chicago，US has found that elephants carry a large number of genes that stop tumors（腫瘤）developing．可知在美國芝加哥大學的一個團隊發(fā)現(xiàn)大象攜帶大量阻止腫瘤發(fā)展的基因．故選D．
33．A推理判斷題．根據(jù)第四段An increased risk of developing cancer has stood in the way of the evolution of large body sizes in many animals，可知在大體型的動物的進化中，患癌癥的風險增加．由此可知，在進化中腫瘤是極大威脅．故選A．
34．B詞義猜測題．根據(jù)第四段But across species，the risk of cancer does not show a connection with body mass．可知物種患癌癥的幾率和它們的體型沒有關(guān)系．故選B．
35．C主旨大意題．根據(jù)最后一段"It's up to us to learn how different animals deal with the problem so that we can use those strategies to prevent cancer in people．"可知是時候了解不同的動物是如何處理這種問題，為了我們可以使用這些策略來預防癌癥．而根據(jù)倒數(shù)第三段In the elephant's case，the making of TP53 is nature's way of keeping this species alive．可知在大象的案例中，TP53的制作是讓這個物種存活的自然方法．由此推知，人們有望通過合成TP53來預防癌癥．故選C．
點評 本文考察學生的理解推斷能力以及細心程度，只要抓住文章的關(guān)鍵詞，采用尋讀的方法查找細節(jié)，就能找到正確答案．
