Hummingbirds(蜂鸟) are one of nature's most energetic fliers and the only birds to hover(盘旋) in the air by relying on their strength alone.

    Now scientists have found that it is the ratio(比值) of the bird's wing length to its width that makes them so efficient. The discovery is helping experts compete with 42 million years of natural selection to build helicopters that are increasingly efficient.

    David Lentink, an assistant professor at Stanford University in California, tested wings from 12 different species of hummingbirds, which he sourced from museums. He placed them on a machine used to test the aerodynamics(气力学) of the helicopter blades(桨叶). Professor Lentink's team used the same machine to test the blades from an advanced micro-helicopter used by the UK's army. They found that the micro-helicopter's blades are as efficient at hovering as the average hummingbirds.

    But while the micro-helicopter's blades kept pace with the average hummingbird wings, they could not keep up with the most efficient hummingbird's wing. The wings of Anna's hummingbird were found to be about 27 percent more efficient than the man-made micro- helicopter's blades.

    While Professor Lentink wasn't surprised at nature's superiority, he said that helicopter blades have come a long way. “The technology is at the level of an average hummingbird,” he said. “A helicopter is really the most efficient hovering device that we can build. The best hummingbirds are still better, but I think it's amazing that we're getting closer. It's not easy to match their performance, but if we build better wings with better shapes, we might match hummingbirds.”

    Professor Lentink said that we don't know how hummingbirds maintain their flight in a strong wind, how they navigate(确定方向) through branches, or how they change direction so quickly. He thinks that great steps could be made by studying wing aspect ratios-the ratio of wing length to wing width. Understanding these abilities and characteristics could be a benefit for robotics and will be the focus of future experiments.

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.

    Emily and her boyfriend had just had a fight. She felt alone and hopeless. Then she went into the kitchen and grabbed what she needed before going back up to her room quietly. She switched on the TV and started eating…and eating…for hours, until it was all gone.

     What Emily didn't know at the time was that she was suffering from an illness called binge-eating disorder(BED)(暴饮暴食).

    For years, Emily didn't tell anyone what she was doing. She felt ashamed, alone, and out of control. Why don't famous people confess (承认) to BED, as they do to anorexia? It's simple: There's a stigma（污名）involved. “Overeating is seen as very bad, but dieting to be skinny is seen as positive and even associated with determination," says Charles Sophy, a doctor in Beverly Hills , California.

    "Some parents or friends may look at a teen with BED and think, 'Oh, a good diet and some will-power will do the trick.' But that's not true," says Dr.Ovidio Bermudez , a baby doctor at the Eating Recovery Center in Denver. "Eating disorders are real physical and mental health issues; it's not about willpower." The focus in treating BED shouldn't be on weight, because as with all eating disorders, the behaviors with food are a symptom of something deeper.

    Like most other diseases, genetics may play a big part in who gets BED and who doesn't. If you have a close relative with an eating disorder, that means you're more likely to develop an eating disorder of your own.

    Besides, many people with BED have tried at some point or another to control it by going on a diet, but paying more attention to food doesn't help. And it might even make things worse, like it did for Carla, who's 15 now and is recovering from BED. "My parents would always tease me about my weight, so when I was 14, I went on a very restrictive diet," she says. When you can't have something, you only want it more, so every time Carla would have a bite of something that wasn't allowed on her strict diet. She would quickly lose control and binge (狂欢).

    From American Express, wedding guests will, on average, spend $673 on each wedding they attend this year. It includes airfare （$225）, hotel （$170）, dining out （$116）and dressing up （$95）and the gift. If you have weddings to attend this year, here are some tips for you to avoid breaking the bank.

    ⒈Book flights in advance

    The moment you decide to attend a wedding is the time to check flight prices at the best time. Plane fares are higher in the summer, especially in July and August. Booking in advance will save you money, as will watch for sales on lower﹣cost carriers like JetBlue and Frontier. You can check Google Flights for a calendar of prices showing the cheapest days to fly from apps like Hopper to get real﹣time alerts when a fare is at its lowest price point.

    ⒉Don't blow your budget on the gift

    If you've got the money, an expensive gift is lovely. But there's no need to take out loans to prove your love for the happy couple. Skip an expensive necklace by giving （an appropriate amount of） cash instead. To save on the gift, consider making one： A photo album or scrapbook of memories with the bride and groom shows how much you care. You could also share the gift with other guest(s) or even make gifts with DIY ideas by yourself to save money.

    ⒊Use old dresses and suits

    You don't always have to be on a new dress for a wedding. While men have the option of repeating their suits, women are more likely to spend money on new clothes for the special occasion. But before you take out your wallet, consider reinventing something already in your closet. Belts, colorful scarves, shoes can bring life to any outfit.

    However exciting space exploration sounds, there's a necessary and important point about it that needs to be considered: food supplies. Right now, astronauts typically rely on dry food in airtight bags and cans, since there are strict weight limits on items taken into space. Foods that we take for granted, such as fresh fruits and vegetables, are out of the question for space explorers.

    For those who've made an effort to try to grow food during space flights, they've faced many difficulties, including the absence of gravity, and a lack of soil, air and humidity (湿度). However, growing food to add and minimize (最小化) the food that must be carried to space will be increasingly important on long-duration flights into space. Great efforts have been made to explore the concept (idea) of space farming. Recently, a team led by Federico Maggi at the University of Sydney in Australia have worked out how plants can absorb nutrients from human urine (尿), as was reported by New Scientist on March 27.

    After over 20 years of experiments, the results suggested that human urine could supply three to four out of the six nutrients that plants need. The researchers also found out that urine-fertilized plants produce no harmful by-products, such as carbon dioxide or ammonia.

    According to New Scientist, human urine is 95 percent water, with the other 5 percent made from nutrients which are harmful to the human body but not to plants. The advantage of this urine-fueled life support system is obvious: By recycling liquid waste and producing food, an efficient cycle will be created.

    And most importantly, said New Scientist，the duration of space flights will be greatly extended to “20 years of flight”, meaning we may be soon sending astronauts on flights to Mars, or even beyond.

    Being stuck at home doesn't mean that learning stops. In fact, it's just the opposite. Here are some activities and resources to foster kids' imagination and build their curiosity at home.

    Let's Get Down to the Science

The science lab at school may be closed, but you can still put on goggles and a white lab coat and learn at home. This Test Tube Experiment Kit will make you feel like a genius in no time. Kids will get the chance to learn about basic chemistry, physics, and more by creating pop-resistant bubbles, bouncy balls, and other objects.

    To Infinity (无穷) and Beyond

Earth is a part of the Milky Way galaxy, but did you know there are around 100 million galaxies in the universe? There are so many fun and fascinating things to learn about space that the possibilities truly go to infinity and beyond. A great way start your child's educational space adventure is with this Real Life Sticker and Activity Book.

    Take a Walk on the Wild Side

    Biology class is in session! Here's your chance to get up close and personal with a great white shark without putting on a wetsuit. This 4D Great White Shark Anatomy Kit allows you to get an inside look at what makes these predators of the deep so powerful. And then take an even deeper dive into a shark's world with I Am Shark.

    Take advantage of this time at home and find new ways to keep exploring the world around you.

Edward Ndiritu joined Lewa Wildlife Reserve in central Kenya in 1996 as a rhino monitor. Back then, illegal hunting had greatly reduced the country's black rhino population. Worried that they might become extinct, he worked his way up to lead Lewa's anti-hunting team. In 2015, his team's community-focused conservation method won him the first Wildlife Ranger Award from conservation charity Tusk.

He grew up in a village between Mount Kenya and the Aberdare Forest, which was once a wildlife route for migrating elephants and black rhinos. He saw them disappear gradually. He realized that hunting and the disruptive actions of the community were the reasons. The demand for rhino horn(角), which is very valuable in Asia, drives hunters. They are part of a big and rich crime network. There are people who help them by giving them equipment. In 2011 and 2012, Lewa had many difficulties. They lost rhinos. It was found that those involved in hunting were also linked to human smuggling(走私) and other illegal activities.

The hunting challenge has changed. Hunters now have networks, get information from the local community, and use technology like phones and motorbikes. Edward's team has adapted by using cameras, sensors, trackers, and a system that tells them when and where hunting might happen. They also work closely with the community, which is now the first 

line of defense. Lewa gives agricultural training, medical help, and business support to rural women. This makes locals more likely to report hunting before it happens.

To be a good forest guard, one needs to be honest, tough, and motivated. Edward now knows that making sure the forest guards are happy and well is very important. He is hopeful about wildlife conservation in Lewa. Funding is a problem, but organizations like Tusk are helping. Conservation is about people. Looking after those outside the reserve is necessary for the future of wildlife. Only when communities can live in harmony with the animals they see every day will there be a good future for wildlife.