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 (壁虱) 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, or get help, 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 of fish trash.
Small birds called plovers are also in the cleaning business.Theyhavebigcustomers,crocodiles.Crocodileshavelongsnouts(口鼻部)filledwithsharpteeth.Cleaningthemistricky.Whenacrocodileopensitsmouth,theploverhopsrightin.Thecrocodiledoesnotsnapitssnoutshut.Instead,itletstheplover eat small, harmful animals attached to its teeth. The plover gets an easy meal while the crocodile gets clean teeth.
1．The text is mainly about ________.
A．the hard life in the wild
B．some dangerous animals
C．animals living together
D．animals in a symbiotic relationship
解析：选D 主旨大意题。本文主要介绍了生物界的一种共生现象，也就是说生物之间相互依存、相互帮助。故答案选D项。选项C表达语意不准确，表达范围太广。
2．According to the text, in a symbiotic relationship, ________.
A．animals depend on each other
B．birds depend on animals for food
C．both sides can get help
D．not all animals treat their partners well
解析：选D 推理判断题。由文章第二、三、四段可知，有共生现象的动物中，有的是相互得到帮助，而有的是单方面的。故答案为D项。
3．When plovers hop into crocodiles' mouths, they ________.
A．are really to clean crocodiles' teeth
B．are to make crocodiles happy
C．aren't afraid of being eaten
D．aren't happy to clean the crocodiles' teeth
解析：选C 细节理解题。由文章最后一段可知，plovers 是不担心被鳄鱼咬伤的，因为鳄鱼是希望 plovers 这样做的，而对于 plovers 而言，其真正的目的是为了得到自己的食物。故答案为C项。
4．According to the text, it can be inferred that ________.
A．living in the wild can be hard
B．different animals have different living ways
C．crocodiles mainly live on plovers
D．ticks are useful to some animals
解析：选B 推理判断题。文章第一段就提到有些动物是靠自己去捕捉食物，而文章又举例讲了一些动物是靠共生来获取食物的，也就是说不同的动物有不同的方式来生存，所以答案为B项。选项A是文章直接叙述的；选项C表达与原文不一致；选项D是无法推断出的。
