In 1999, twelve percent of public elementary schools in the United States required students to wear uniforms. Just three years later, the amount was almost double that.

    A study of six big-city Ohio public schools showed students who were required to wear uniforms had improved graduation, behavior and attendance rates. Academic performance was unchanged.

    Some high schools in Texas have also joined in the movement. Yet studies find mixed results from requiring uniforms. And some schools have turned away from such policies.

    Supporters believe dressing the same creates a better learning environment and safer schools. The school district in Long Beach, California, was the first in the country to require uniforms in all elementary and middle schools. The example helped build national interest in uniforms as a way to deal with school violence and improve learning.

    Findings in Long Beach suggested that the policy resulted in fewer behavior problems and better attendance. But researcher Viktoria, who has looked at those findings, says they were based only on opinions about the effects of uniforms.

    She says other steps taken at the same time to improve schools in Long Beach and statewide could have influenced the findings. The district (the area marked by government) increased punishments for misbehavior. And California passed a law to reduce class sizes.

    In Florida, for example, researcher Sharon found that uniforms seemed to improve behavior and reduce violence. In Texas, Eloise found fewer discipline problems among students required to wear uniforms, but no effect on attendance.

    Sociologist David has studied school uniform policies since1998.He collected the reports in the book. In his own study, he found that reading and mathematics performance dropped after a school in Pennsylvania（宾夕法尼亚州）required uniforms.

    Political and community pressures may persuade schools to go to uniforms to improve learning. But David and others believe there is not enough evidence of a direct relationship. In fact, he says requiring uniforms may even increase discipline problems.

根据短文内容，选择最佳答案，并将选定答案的字母标号填在题前括号内。

    A black hole is created when a large star burns out. Like our sun, stars are unbelievably hot furnaces(熔炉) that burn their own matter as fuel. When most of the fuel is used up, the star begins to die.

    The death of a star is not a quiet event. First there is a huge explosion. As its outer layer is blasted off into space, the dying star shines as brightly as a billion suns.

    After the explosion, gravity pulls in what's left of the star. As the outside of the star sinks toward the center, the star gets smaller and smaller. The material the star is made of becomes tightly packed together. A star is so solid that a teaspoon of matter from it weighs billion of pounds.

    The more the star shrinks(收缩), the stronger the gravity inside it becomes. Soon the star is very tiny, and the gravity pulling it in is unbelievably strong. In fact, the gravity is so strong that it even pulls light into the star! Since all the light is pulled in, none can go out. The star becomes black when there is no light. Then a black hole is born!

    That's what we know about black holes. What we don't know is this: What happens inside a black hole after the star has been squeezed into a tiny ball? Does it keep getting smaller and smaller forever? Such a possibility is hard to imagine.

    But if the black hole doesn't keep shrinking, what happens to it? Some scientists think black holes are like doorways to another world. They say that as the star disappears from our universe, it goes into another universe. In other words a black hole in our universe could turn into a "white hole" in a different universe. As the black hole swallows(吞噬) light, the white hole shines brightly--somewhere else. But where? A different place, perhaps, or a different time--many years in the past or future.

    Could you travel through a black hole? Right now, no. Nothing we know of could go into a black hole without being crushed(挤压). So far the time being, black hole must remain a mystery.

    Black holes are a mystery--but that hasn't stopped scientists from dreaming about them. One scientist suggested that in the future we might make use of the power of black holes. They would supply all of Earth's energy needs, with plenty to spare. Another scientist wondered if a black hole could some day be used to swallow earthly waste--a sort of huge waste disposal(处理) in the sky!

    Superman, Spider-Man, Batman and Iron Man ... .There is no shortage of superheroes. You find them in comic books, on TV and the big screen.

    In a survey of fans by the sci-fi（科幻）and fantasy website, SFX.co.uk, Batman, 70 years old this year, was voted Britain's favorite superhero.

    Unlike many crime-fighting superheroes Batman has no special powers. He can't fly like Superman or shoot a sticky web like Spider-Man.

    Bruce Wayne is born to a wealthy Gotham City business family. However, when his parents are murdered his life changes completely.

    Wayne feels angry at their deaths and guilty that he did not prevent them. He travels around the world learning how to fight. Upon returning to Gotham, he creates a disguise (伪装) to enable him to fight crime without being recognized.

    A childhood fear of bats leads him to choose to dress as one. His idea is that through the bat persona (角色) he can prove to himself that he has overcome his childhood fears.

    Wayne is the CEO of the company he inherits from his father. He seems to live the lifestyle of a millionaire playboy. But this is a ruse (计策). He works hard at the image to allow himself the freedom he needs to do his work as a crime fighter. "Bruce Wayne, playboy" is the disguise; Batman is the real person.

    "Wayne is not a born superhero. Instead, he is a real, complex person," said Dace Golder, editor of the website. "He is the most realistic of all the superheroes. I am particularly interested in the emotional process by which a boy becomes a hero. His superhero qualities come from within."

    My daughter Kelly is a cautious person. She needs to warm up to situations, and is hesitant to try new things. When with close friends, she becomes a leader who laughs loudly and chants, "Girls rule, boys drool." But when that comfort zone is not around her, she is shy and nervous.

    This has been challenging for me at times. "Shy" is not a word that I think has ever been used to describe me. But this has been a year of firsts for my girl that has filled her with a new sense of confidence. This year she moved to lap lane in swimming where she was preparing for a swim team. This year she learned to ride a bike without training wheels. And this year she completed her first kids' triathlon(三项全能).

    On Saturday, with a thunderstorm coming soon and my son's birthday party later in the day, we all went out in the dark of the morning for Kelly to participate in her first triathlon. We practiced transitions from swim to bike to run with her, we got all the equipment she'd need, and we kept talking about the race. But as we waited the two hours for the older kids to finish before her turn, she held my leg a little harder and told me she loved me a few too many times. She was nervous but trying to keep it together.

    And then it was her turn. From the second she jumped into the water, my heart soared. My daughter transformed into the most confident human being I had ever seen. She dominated that swim, crushed that bike ride and ran to the finish with the biggest smile on her face.

    I can honestly say that I never felt so proud of someone in my entire life. It wasn't because she did a sport or anything like that. It was because she was afraid of something and conquered that fear with confidence and a fire I hadn't seen before.

    All day I would find myself just looking over at her and smiling. She might be wearing the finalist medal but I felt like I won that day. I won the chance to see my girl shine.

    Shine on, sweet baby.

    When my daughter Sara was in the fifth grade, she came to me with a problem." Marcy hates me！" she cried." Because Kathy is my friend, too. She wants me to be her friend and nobody else's. You talk to Marcy. You tell her that I want to be her friend, but I can have other friends, too！"

    Oh！I looked at her for a few moments，wondering how I got into this mess，when suddenly an idea came to me.

    Picking up two baskets from the living room, I explained, "When everyone is born, he or she has a little basket. This little one here is yours. The big one is mine. As you grow, so does the basket. You can see your little basket is inside mine because when you were born, there were too many things you couldn't do for yourself. I did everything you couldn't do on your own." She nodded.

    "Well, as you grew older and began to do some things on your own, I began placing a few more things in your basket. When you learned to tie your shoes, that went in your basket."

    She said softly, "I can tie my own shoes."

    "Right. As you grow older，there will be more and more things you must do on your own." As I spoke, I gradually took her basket out of mine and handed it to her." You will finally carry your own basket with things only you can do."

    She looked up at me and said," I understand. There are some things that I have to do for myself because they are in my basket."

    If you follow these insider tips, your photos will look like you had the place to yourself at the popular tourist attractions.

    Vatican Museums: Vatican City

    The Vatican draws more than five million people each year, and queues can reach four hours during peak season. Christie Hudson, senior communications manager at Expedia, recommends, choosing a skip-the-1ine tour. "This not only lets you avoid the ticket counter, but also includes the use of a private partner entrance." Extra time to visit the Sistine Chapel? Yes!

    Bamboo Forest: Kyoto, Japan

    Bamboo Forest is the most worthy sight in Kyoto. If you're longing to enjoy the pathways and take pictures in total quietness, Kyoto Arashiyama Travel Guide recommends hitting the famous Bamboo Grove Path as early in the morning as possible—think 7 a.m. if you're up for it. Don't miss these hidden treasures you can only witness in Japan.

    Chichen Itza: Yucatán, Mexico

    Home to E1 Castillo and the Temple of the Warriors, Chichen Itza is a must-see. Want to beat the rush? Schedule an early tour that takes place before a site opens to the public. Led by an archaeologist guide, it's full of fascinating insights and facts—without tons of pack-wearing tourists.

    Louvre Museum: Paris, France

    The Louvre is one of the most popular museums on the planet. If waiting around in line to get in isn't the way you prefer to spend your time in Paris, consider purchasing a reserved ticket. This will give you entry to the pyramid within a half-hour window. The Louvre is also open until 9:45 p. m. on Wednesdays and Fridays, if you're up for some late-night art visits.