More visitors to Beijing since the repair project was completed．( )A．were broughtB．are broughtC．will be broughtD．have been brought 题目和参考答案——青夏教育精英家教网——

题目内容

7．More visitors ___________ to Beijing since the repair project was completed．（　　）

	A．	were brought		B．	are brought
	C．	will be brought		D．	have been brought

分析自从修缮工程完成后，更多的游客被带到了北京．

解答答案是D．考查被动语态，主语visitors与动词bring 之间是被动关系，since引导的时间状语中主句要用have+过去分词，所以答案选D．

点评被动语态由"助动词be+及物动词的过去分词"构成．被动语态的时态变化只改变be的形式，过去分词部分不变．疑问式和否定式的变化也如此．掌握各种时态的被动语态是解题的关键．

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17．Back in the 1860s，a British scientist named Henry Walter Bates noticed something interesting in the animal world：some harmless animals change the color or patterns of their skin to pretend to be deadly．For example，a kind of butterfly，Common Mormon，can change the pattern on its wings so that it looks like another butterfly species，the Common Rose，which is poisonous for birds to eat．
    Bates argued that animals develop this ability to protect themselves from being eaten．This theory had been widely accepted by scientists．But one question remains：how do these animals manage to do that？
    After more than 150 years，scientists are finally able to answer that question-it is all down to a gene called"doublesex"，according to a study published on March 6 in the journal Nature．
    In fact，scientists have long known that genes are responsible for this useful ability．Just like your hair color and the shape of your eyes are determined by your genes，the patterns on a butterfly's wings can be explained similarly．But until the new study，scientists hadn't been able to find out which genes in particular were responsible．
    Encouraged by the development of new technologies for studying genes，scientists at the University of Chicago，US decided to give it another try．They compared the genetic structure of the Common Mormons that changed their wing patterns with those of ones that did not．And all the results pointed to a single gene，"doublesex"．
This was much to the scientists'surprise．They used to expect that something as complex as this would be controlled by many different genes．For example，one gene would control the color of the upper part of the wing，one would do so for the lower part and one would control wing shape．
    However，the"doublesex"gene doesn't quite do everything by itself．Instead，it acts like a switch-it"tells"other genes to change the wing patterns．Isn't that clever？
    Scientists believe that this special ability of the"doublesex"gene was developed throughout the long history of evolution．"The harmless species gains an advantage by resembling something predators （捕食动物） avoid，"Sean Carroll，a scientist at the University of Wisconsin-Madison，US told Nature．"It was obvious evidence for natural selection．"
63．What was newly discovered by scientists at the University of Chicago？D
A．Some animals can change their skin color．
B．Some butterflies can disguise themselves as other species．
C．Genes are responsible for the change in animals'skins．
D．Common Mormons change their skin patterns due to a certain gene．
64．What surprised scientists in the study？C
A．So many genes are involved in the change of patterns．
B．The genetic structure of the butterflies is so complex．
C．A single gene determines the change of patterns．
D．The gene"doublesex"singlehandedly changes the wing patterns．
65．Which of the following is the best title？B
A．A new discovery
B．How butterflies stay safe
C．A gene called"doublesex"
D．Natural selection．

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