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    Living in the wild can be hard. Finding food and staying safe aren't easy. Each day, animals struggle to survive in their habitats. Not all animals get by on their own. Some animals form a close partnership with other kinds of animals. These pairings are called symbiotic (共生的) relationships.

    In a symbiotic relationship, the animals depend on each other. One animal helps the other meet its needs. Sounds good, right? Not always. Some animals are not very kind to their partners. In some cases, one animal meets its needs but hurts its partner. Take ticks, for example. These insects suck blood to live. To get blood, they attach themselves to other kinds of animals. Ticks can pass germs that cause disease instead of helping their hosts.

    In other relationships, animals don't treat their partners so poorly Both animals benefit from living with the other animal.

    Small animals called cleaner shrimps have found a way of helping fish at coral reefs. As their name suggests, the shrimps clean the fish. They hang out at what scientists call a cleaning station. A fish stops by. Then a shrimp climbs onto the fish and even steps into the fish's mouth. The shrimp uses its tiny claws to pick stuff off the fish's body. That can include dead skin, tiny pieces of food, and wee creatures that can hurt the fish. The fish gets a nice cleaning. The shrimp enjoys a tasty meal offish trash.

    Small birds called plovers are also in the cleaning business. They have big customers—crocodiles. Crocs have long snouts（鼻子）filled with sharp teeth, Cleaning them is tricky. When a croc opens its mouth, the plover hops right in. The croc does not snap its snout shut. Instead, it lets the plover eat small, harmful animals attached to its teeth. The plover gets an easy meal while the croc gets clean teeth.

根据短文内容，请将单词填写在题号对应的横线上。

    Cheating happens a lot in our city. It can be hard to know who is really in 1 and who is trying to take advantage of other people's   2.

    Recently, I was finishing my work at home3 the dog barked letting me know that 4 was at the gate. I rushed to see who it was and there I saw a 5 weak woman in her late fifties.

    I said to her, “May I ask what you6?”

    She 7, “Hello, my name is Parvathy. My husband and I live nearby. We earn our living8working at construction sites. A few days ago my9 fell off the stairs while 10 a heavy load. He is 11hurt and hasn't been able to 12for the past five days, so we have no food. Now he is at home in bed with a high fever. Please help me! I need to take my husband to a doctor!”

    “Oh!” I said. “Please13here.” I went into the room. I felt really bad about her 14but ...Was she telling me the15or trying to cheat me out of my money?

    Finally, I 16 to help her. But first I said a simple prayer, “Dear Lord, you know17 I earn my pennies, so please make sure that my hard earned money helps those who 18 need it.”

I went out with a fifty note and19 it to her. The moment she got the money she broke down in tears. She took my hand and said, “Thank you so much!”

    I was so 20, and turned back to my work smiling.