Our sun is a star, and nine worlds (天体) move round it. Worlds like these are planets. But here are other stars, other suns, not only thousands of them, but millions of them. Do different stars have planets? We can see the planets round our own sun. But we haven't been anymore. The stars are too far away from us. Stars give out light, but the planets don't have any light of their own. So we can't see them. They are too far, and too dark. Still, we can tell that some stars have planets. While a big planet goes round, it pulls its star to one side. And sometimes we can see this. Then we know that the star has a planet. For example, Barnard's Star is a small red star. And it is the quickest star in the universe. But sometimes it moves slower, and then quicker again. We have known Bernard''s Star for many years, and now we are sure it has a planet.

阅读下面短文，从A、B、C、D四个选项中选出最佳选项。

    Almost everybody in America will spend a part of his or her life behind a shopping cart. They will, in a lifetime, push the shopping carts many miles, but few will know—or even think to ask—who it was that invented them.

    Sylvan N. Goldman invented the shopping cart in 1937. At that time he was in the supermarket business. Every day he would see shoppers lugging groceries around in baskets they had to carry.

    One day Goldman suddenly had the idea of putting baskets on wheels. To create the first shopping cart, Goldman used folding chairs. He out one basket on the seat, and he raised the chair by putting wheels under the legs. The wheeled baskets would make shopping much easier for his customers, and would help to attract more business.

On June 4, 1937, Goldman's first carts were ready for use in his market. He was terribly excited on the morning of that day as customers began arriving. He couldn't wait to see them using his invention. However, Goldman was disappointed. Most shoppers gave the carts a long look, but hardly anybody would give them a try. After a while, Goldman decided to ask customers why they weren't using his carts.

    "Don't you think this arm is strong enough to carry a shopping basket?" one shopper replied.

    Goldman wasn't beaten yet. He knew his carts would be a great success if only he could persuade people to give them a try. To this end, Goldman hired a group of people to push carts around his market and pretend they were shopping! Seeing this, the real customers gradually began copying the phony customers.

As Goldman had hoped, the carts were soon attracting larger and larger numbers of customers to his market. Not only did more people come—those who came bought more. With larger, easier-to-handle baskets, customers unconsciously bought a greater number of items than before.

    Today's shopping carts are five times larger than Goldman's original model. Perhaps that's one reason Americans today spend more than five times as much money on food each year as they did before 1937—before the coming of the shopping cart.

    Have you ever heard someone use the phrase "once in a blue moon"? People use this expression to describe something that they do not do very often. For example, someone might say that he tries to keep away from sweets because they are unhealthy, but will eat chocolate once in a blue moon. Or someone who does not like to. Go to the beach might say, "I visit the shore(岸边) once in a blue moon." While many people use this phrase, not everyone knows the meaning behind it.

    The first thing to know is that the moon itself is never actually blue.  This is just an expression. The phrase "blue moon" actually has to do with the shape of the moon, not the color.

    As the moon travels around the earth, it appears to change shape. We have certain names for certain shapes of the moon. For example, when we can see a small part of the moon, it is called a crescent moon. A crescent is a shape that looks like the tip of a fingernail(指甲). When we can see the moon appear in the sky as a thin crescent, it is called a new moon. When we can see the whole moon, it is called a full moon. Usually, there is only one full moon every month. Sometimes, however, there will be two full moons in one month. When this happens, the second full moon is called a "blue moon".

    It's predicted(预言) that there will only be 15 blue moons over the next 20 years. As you can see, a blue moon is a very unusual event. This fact has led people to use the expression "once in blue moon" to describe other" very unusual events in their lives.

Zhou Dao's grandfather has a hearing problem. During a family party last year, Zhou Dao noticed something. His grandfather sat quietly by himself. The old man was unable to join the talks.

Zhou Dao was sad about this. "Grandpa always reads subtitles (字幕) to watch TV," he thought. "Why can't we put words on our talks?" Then, Zhou Dao began working on that idea. He spent about half a year developing a kind of smart glasses. These smart glasses help users "read" spoken words.

The glasses are connected to a smartphone. There is a microphone on the glasses. It records audio (录制音频) and sends it to the phone. A piece of software turns the audio into words. People can see the words on the glasses.

This allows users with hearing problems to understand others during the talks well. Now Zhou Dao's grandfather can wear the glasses and speak to someone else face to face or over the phone.

Zhou Dao was happy to help his grandfather. Slowly, more and more people in need can wear this kind of glasses.