When we see well, we do not think about our eyes very often. It is only when we cannot see perfectly that we realize how important our eyes are.

    People who are near-sighted can only see things that are very close to their eyes. Everything else seems ­­blurry. Many people who do a lot of work, such as writing, reading and sewing become near-sighted. People who are far-sighted suffer from just the opposite problem. They can see things that are far away, but they have difficulty in reading a book unless they hold it at arm's length. If they want to do much reading, they must get glasses, too.

    Other people do not see clearly because their eyes are not exactly the right shape. They have what is called astigmatism (散光). This, too, can be corrected by glasses. Some people's eyes become cloudy because of cataracts (白内障). Long ago these people often became blind. Now, however, it is possible to operate on the cataracts and remove them.

    Having two good eyes is important for judging distances. Each eye sees things from a slightly different angle. To prove this to yourself, look at an object out of one eye; then look at the same object out of your other eye. You will find the object's relation to the background and other things around it has changed. The difference between these two different eye views helps us to judge how far away an object is. People who have only one eye cannot judge distances as people with two eyes.

阅读下列短文，从每题所给的四个选项 （A 、B 、C  和 D ）中，选出最佳选项。

    A recent announcement by scientists that they have successfully cloned the first human embryo(胚胎) has caused much debate and has shocked many people around the world. On the one hand, some scientists point out that if you clone an embryo, you can produce valuable tissues(组织) and organs that could be used to save human lives. On the other hand, many people, including some scientists, disagree and fear that if mankind interferes with (干涉) nature in this way, they may be on their way to producing a real-life Frankenstein's monster.

Cloning is producing an exact copy of a plant or an animal using its cells. The first mammal to be cloned successfully from an adult cell was Dolly—the sheep. She was born in 1996 and died in early 2003, at a much younger age than normal. When she was born, many people were angry because they thought cloning would create more diseases in the animal world. However, in general the scientists were praised for their wonderful scientific breakthrough.

    The Scottish scientist who created Dolly, Ian Wilmut, is shocked that some scientists are now considering cloning human beings. Although he researches cloning, he has never thought of creating copies of humans. Instead, he thinks the scientists should concentrate on creating new tissues and organs that could eventually be used to cure diseases like cancer. However, some people consider that cloning human embryos with the intention of destroying them shows no respect for human lives.

    While cloning human embryos is illegal in many countries, some scientists are already pushing ahead with the research so as to deliver a cloned human baby. Severino Antinori, an Italian doctor, is one of the leaders in this field of research. He has declared that he wants to be the first to clone a human being.

In China, scientists have focused their efforts on cloning animals, as well as stem cells to be used in medical research. China has succeeded in producing clones of cows and goats, and continues to research ways in which cloning can benefit mankind.

    What makes a gift special? Is it the price you see on the gift receipt? Or is it the look on the recipient's face when they receive it that determines the true value? What gift is worth the most?

    This Christmas I was debating what to give my father. My dad is a hard person to buy for because he never wants anything. I pulled out my phone to read a text message from my mom saying that we were leaving for Christmas shopping for him when I came across a message on my phone that I had locked. The message was from my father. My eyes fell on a photo of a flower taken in Wyoming, and underneath a poem by William Blake. The flower, a lone dandelion standing against the bright blue sky, inspired me. My dad had been reciting those words to me since I was a kid. That may even be the reason why I love writing. I decided that those words would be my gift to my father.

    I called back. I told my mom to go without me and that I already created my gift. I sent the photo of the cream-colored flower to my computer and typed the poem on top of it. As I was arranging the details another poem came to mind. The poem was written by Edgar Allan Poe; my dad recited it as much as he did the other. I typed that out as well and searched online for a background to the words of it. The poem was focused around dreaming, and after searching I found the perfect picture. The image was painted with blues and greens and purples, twisting together to create the theme and wonder of a dream. As I watched both poems passing through the printer, the white paper coloring with words that shaped my childhood, I felt that this was a gift that my father would truly appreciate.

    Christmas soon arrived. The minute I saw the look on my dad's face as he unwrapped those swirling black letters carefully placed in a cheap frame, I knew I had given the perfect gift.

    It may sound strange, but cutting down a real tree for Christmas is actually greener than going with the artificial kind, one scientist says.

    “It is a little confusing to people,” said Clint Springer, a biologist at Saint Joseph's University in Philadelphia. Because of concerns over deforestation（砍伐森林）around the world, many people naturally worry that buying a real tree might contribute to that problem. But most Christmas trees for sale these days are grown not in the forest but on tree farms for the purpose of being cut.

    Moreover, from the viewpoint of greenhouse gases, real trees are “the obvious choice”. Live trees actively release oxygen as they grow, and meanwhile remove carbon dioxide from the atmosphere. After they have been cut and Christmas is over, they're usually cut into smaller pieces for mulch（覆盖物）. As mulch, the bits of trees very slowly release carbon dioxide back into the atmosphere. So in the end, a real Christmas tree is carbon neutral, putting the same amount of carbon dioxide back into the air as it took out (although much more slowly).

    The tree farms that grow the trees also replant after the trees are cut.  Artificial trees, on the other hand, don't come out even in the carbon balance. Petroleum（石油）is used to make the plastics in artificial trees and lots of carbon dioxide-creating energy is required to make and transport them. Because these trees just end up in the places where waste is buried under the ground after use, “those greenhouse gases are lost forever,” Springer said.

    Reading may be fundamental, but how the brain gives meaning to letters on a page has been a mystery. Two new studies fill in some details on how the brains of efficient readers handle words. One of the studies, published in the April 30 Neuron, suggests that a visual-processing area of the brain recognizes common words as whole units. Another study, published online April 27 in PLOSONE, makes it known that the brain operates two fast parallel systems for reading, linking visual recognition of words to speech.

    Maximilian Riesenhuber, a neuroscientist at Georgetown University in Washington, D.C., wanted to know whether the brain reads words letter by letter or recognizes words as whole objects. He and his colleagues showed sets of real words or nonsense(无意义的词语)words to volunteers undergoing fMRI scans. The words differed in only one letter, such as “farm” and “form” or “soat” and “poat”, or were completely different, such as “farm” and “coat” or “poat” and “hime”. The researchers were particularly interested in what happens in the visual word form area, or VWFA, an area on the left side of the brain just behind the ear that is involved in recognizing words.

    Riesenhuber and his colleagues found that neurons(神经元)in the VWFA respond strongly to changes in real words. Changing “farm” to “form”, for example, produced as great a change in activity as changing “farm” to” coat”, the team reports in Neuron. The area responded slowly to single-letter changes in made-up words.

    The data suggests that readers grasp real words as whole objects, rather than focusing on letters or letter combinations. And as a reader's exposure to a word increases, the brain comes to recognize the shape of the word. Meaning is passed on after recognition in the brain, Riesenhuber says.

    The researchers don't yet know how longer and less familiar words are recognized, or if the brain can be trained to recognize nonsense words as a unit.

    A single dose(剂量) of an experimental vaccine(疫苗) can protect mice against the Zika(寨卡) virus, raising renewed hope of a vaccine for humans, say scientists. The US team say the results, published in Nature, are "striking" and should encourage research efforts.

    Tests in humans could begin in months. But even if these go well, a licensed vaccine for widespread use to protect those at most risk ­ such as pregnant women ­ would still be years away, experts advise. Zika has been spreading across Central and South America and, most recently, Africa. More than 60 countries and territories now have continuing transmission(传播) of the disease, which is carried by mosquitoes. The virus causes serious birth damages during pregnancy and has been declared a global public health emergency.

    Now, developing a vaccine for pregnant women to protect their unborn babies is an international research priority(优先事项). US scientists from the Walter Reed Army Institute of Research, the Beth Israel Deaconess Medical Center and Harvard Medical School tested two types of Zika vaccine in mice – one based on bits of genetic(基因的) code from the virus and another that is an inactive (and therefore harmless) copy of Zika. Both worked well, protecting every mouse that was immunised against the virus. In comparison, all of the mice not given the vaccine caught Zika after they were exposed to it.

    Researchers say they will push ahead with developing the needed virus vaccine. There are many existing vaccines for other disease that use this type of technology, while there are relatively few DNA­based vaccines. Of course, future tests will need to check the vaccine is safe and effective in humans, as well as how long the immunity might last.