根据短文内容，从短文后的选项中选出能填入空白处的最佳选项，选项中有两项为多余选项。

      Secret codes (密码)keep messages private。Banks, companies, and government agencies use secret codes in doing business, especially when information is sent by computer.

     People have used secret codes for thousands of years. {#blank#}1{#/blank#}  Code breaking never lags(落后) far behind code making. The science of creating and reading coded messages is called cryptography.

     There are threemain types of cryptography. {#blank#}2{#/blank#}  For example, the first letters of “My elephant eats too many eels” Spell out the hidden message “Meet me.”

{#blank#}3{#/blank#} You might represent each letter with a number, For example, Let's number the letters of the alphabet, in order, from 1 to 26. If we substitute a number for each letter, the message “Meet me” would read “13 5 20 13 5.”

     A code uses symbols to replace words, phrases, or sentences. To read the message of a real code, you must have a code book. {#blank#}4{#/blank#} For example “bridge“ might stand for “meet” and “out” might stand for “me.” The message “bridge out” would actually mean “Meet me.” {#blank#}5{#/blank#}  However, it is also hard to keep a code book secret for long. So codes must be changed frequently.

A. It is very hard to break a code without the code book.
B. In any language, some letters are used more than others.
C. Only people who know the keyword can read the message.
D. As long as there have been codes, people have tried to break them.
E. You can hide a message by having the first letters of each word spell it out.
F. With a code book, you might write down words that would stand for other words.
G. Another way to hide a message is to use symbols to stand for specific letters of the alphabet.

    One Sunday, my son asked me if he could ride up to his elementary school on his bike and meet his friend. He wanted both of them to ride back to our house so they could play video games and jump on the trampoline (蹦床). I have to admit, part of me wanted to say no. We could go to pick him up or his parents could bring him over here. I thought. But my son is eleven years old now. And after all, I do let him ride his bike to school. But I also drive my daughter to school and I can see him on the way, making sure he is getting there safely.

    My husband thinks I am overprotective. I don't dare to let my children walk anywhere without one of us going along. As you go out of our neighborhood, there is a shopping center across the street. My son always asks if he can ride his bike or walk over to the drugstore by himself. But crossing that street is just too dangerous. The cars fly around the comer like they're driving in a car race. What if he gets hurt? What if some teenager bullies are hanging out in the parking lot? I want so much to give my children the freedom that I enjoyed having when I was growing up but I hesitate to do so, because there are dangers around every comer. Too many kidnap, too many robberies and so on.

    I honestly don't think my mom worried about such things when her children were young. Growing lip in the 1970s was indeed very different. I never wore a helmet (头盔) when I rode a bike. We were all over the neighborhood, on our bikes and on foot, coming home for dinner and then-back out again until dark. We rode in the back of the truck and didn't wear seat belts. I walked to and from school every day.

    If you want to become a fluent English speaker you should take some advice. There are four skills in learning English. They are reading, listening, speaking, and writing. The most important thing you must remember is that if you want to improve your speaking and writing skills, you should first master the skills of reading and listening.

    Read as much as you can. But your reading must be active. It means that you must think about the meaning of the sentence, the meaning of the unfamiliar（不熟悉） words, etc. There is no need for you to pay much attention to grammars or try to understand all the unfamiliar words you come across, but the fact that you see them for the first time and recognize them whenever you see them, for example, in other passages or books, is enough. It would be better to prepare yourself a notebook so you can write down the important words or sentences in it.

    As for listening, there are two choices: besides reading, you can listen every day for about 30 minutes. You can only pay attention to your reading and become skillful at your reading, then you can catch up on your listening. Since you have lots of inputs in your mind, you can easily guess what the speaker is going to say. This never means that you should not practice listening.

    For listening you can listen to cartoons or some movies that are specially made for children. Their languages are easy. Or if you are good at listening you can listen to VOA or BBC programs every day. Again the thing to remember is being active in listening and preferably taking some notes.

    If you follow these pieces of advice, your speaking and writing will improve quickly, and you can be a fluent English speaker one day.

Tour in the Museum

    Experts or folk scholars lead the tour in the museum and throw further light on certain exhibits. Lectures and interactive events will be held to discuss popular scientific topics. Two public themed lectures will be held every month, like "Unlock the Space Station" and "Looking into the Amazing Brain" etc.

    Fan Workshop

    The series of hands-on courses include "WEDO2.0 Robot", "First Step for Robot Design", "Get Creative with Lego", "Space Exploration" and "Science and Technology" etc. Through exploration, cooperation, creation and sharing, the workshop is set to interest students in science, engineering and mechanics.

    Everyone Can Code

    Based on Apple's Swift language and Swift Playgrounds, the courses, such as How an app is produced, Exploration into Face ID and Program for Dash robot are available to fuel teenagers' interest in programming and cultivate their logical thinking. The aim is get people to feel the fun in programming.

    Live Show

    Unlike the traditional teaching mode, a scientific show will present the scientific principles. The public is able to see the show as well as be in the show to feel the charm in a closer place. Fourteen topics are now available, including "the Power of Structure", "Voice and Music" and "Secret for a Polar Bear to Keep Warm".

    Most dog owners believe their dogs experience guilt, but animal behaviorists say the "guilty look" of a dog is simply a reaction to you and that dogs lack the ability to feel shame.

    Alexandra Horowitz, a psychology professor and principal investigator at the Horowitz Dog Cognition Lab at Columbia University once carried out a study on dog "guilt". She videotaped 14 dogs in a series of trials and observed how they reacted when their owners left the room after instructing them not to eat a treat. When the owner was gone, Horowitz gave some of the dogs the forbidden treat. Then she would tell the owners whether their dogs had eaten the treat or had behaved. However, Horowitz wasn't always honest with them.

    Horowitz found that the dogs' "guilty looks" had little to do with whether they'd eaten the treat or not. In fact, dogs that hadn't eaten it but were scolded by misinformed owners tended to exhibit the most elements of the "guilty look".

Horowitz says this shows that the dogs' body language is actually a response to their owner's behavior—not an experience of shame for a misdeed. Why then, do dogs look so ashamed when we scold them?

Maybe that look of guilt is the result of a learned association. When you scold your dog for chewing up a pair of slippers, he quickly learns that if he lowers his head and tucks his tail, the undesirable response—raised voice and angry expression—is more likely to disappear.

A team of biologists (生物学家) recently studied wildlife in the forests of northern Ecuador. While doing so, they made a surprising discovery: a Mindo harlequin toad (明多斑足蟾). The creature hadn't been seen alive in 30 years. The scientists couldn't believe their eyes. "It took our brains a while to believe it was a Mindo harlequin toad," says Melissa Costales, a conservation biologist from University of New Brunswick.

The Mindo harlequin is the latest harlequin toad species "to come back from the dead," says Costales. Since 2003, eight others have been found, three of them in Ecuador.

Until recently, 13 of the 25 species of harlequin toads in Ecuador had gone unseen since the 1980s or early 1990s. Scientists thought most of them had been killed by a disease called chytrid (壶菌). This illness is especially harmful to the harlequin toad.

Costales says the Mindo harlequin toad may have developed a resistance (抵抗力) to the disease. That would explain the toad's reappearance. And it could spell good news for other harlequins. Since discovering the first one, Costales's team has found five more. They were all tested for chytrid. None had the disease. But that doesn't mean the survival of the species is guaranteed (保证), Costales says. The harlequin toad is still endangered.

Costales is developing a conservation plan with a zoology museum in Ecuador. She wants to make sure the Mindo harlequin toad doesn't fall back into extinction. "Each rediscovery gives us a second chance to develop better conservation plans," she says. "Not every day do we have the chance to rediscover a species that we believed to be extinct."