题目

在SO42﹣中S显 价，保持二氧化碳化学性质的最小粒子是 ． 答案：【考点】有关元素化合价的计算；分子的定义与分子的特性． 【专题】化学式的计算；物质的微观构成与物质的宏观组成． 【分析】根据原子团中各元素的化合价总和等于原子团的化合价，分子是保持物质化学性质的一种粒子分析． 【解答】解：硫酸根显﹣2价，原子团中各元素的化合价总和等于原子 It is hard to believe that a fish less than 30 centimeters long can be dangerous to people and to other sea animals. Yet the piranha(水虎鱼)swims in the dark waters of South America, ready to bite whenever it is hungry. Many fish eat plants, but the piranha is one of a small group that eats meat. With its sharp teeth and strong jaws, the piranha most often kills other fish. Sometimes, however, it attacks people. A person will not die from a single bite, but the blood from the wound attracts other piranhas. Soon a school of as many as a thousand fish gather by the victim. They all begin to bite. In minutes, only a skeleton(骨架)is left. Perhaps the strangest thing about piranhas is that they are not always dangerous. People may safely swim near them in one river, while in another river only a few miles away, may be attacked. Until several years ago, you could buy piranhas in the United States pet stores. Now there is a law against bringing piranhas into the country to sell them. Many people were afraid that the fish might get loose in lakes or streams. The underlined word "school" probably means "________". A. a place to learn knowledge   B. a large group    C. a long river    D. small pupils Why is there a law against bringing piranhas into the United States? A. Because there are enough piranhas in lakes already. B. Because piranhas are not safe to eat. C. Because piranhas die too easily. D. Because piranhas might get loose and attack swimmers. What is the main idea of the passage? A. Piranhas make good pets.                 B. Piranhas are 30 centimeters long. C. Piranhas swim in South American waters.    D. Piranhas can be dangerous.