| 下列集合A到集合B的对应f是映射的共有几个（ ） ①A={-1，0，1}，B={-1，0，1}，f：x→y=x2； ②A={0，1}，B={-1，0，1}，f：x→y=
③A=R，B=R，f：x→y=
④A={x|x是衡水中学的班级}，B={x|x是衡水中学的学生}，对应关系f：每一个班级都对应班里的学生．
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根据映射的概念：对于集合A中的每一个元素在集合B中都有唯一的元素与它对应，是映射．
考察①②③④四个集合A到集合B的对应f：
对于①、②，对于集合A中的每一个元素在集合B中都有唯一的元素与它对应，它们都是映射；
对于选项③，集合A中的元素0在集合B中没有元素对应，不是映射；
对于选项④，由于衡水中学的学生可以在衡水中学的不同的班级，故集合B中的元素可能在集合A中有几个元素对应，不是映射；
∴是映射的共有2个．
故选B．
| 任务型阅读。用方框中所给单词的适当形式填空，每个单词限用一次，使短文通顺，正确连贯。 | |
because most 2______ love home cooking! The food tastes good and warm, and a family meal brings everyone 3______. In some families, meals are often the only times everyone 4______ one another at the same time. Another reason people enjoy home cooking is that it is often a way of 5______ love. A parent who makes some cookies is not just satisfying a 6______ sweet tooth. She or he is sending a message. The message says, "I care about you enough to spend an hour making cookies that you will eat up 7______ fifteen minutes if I let you." There is also something about the smell of home cooking. The smell of home cooking pleases people of 8______ ages. It makes most of us feel good and 9______, even if we are the ones doing the cooking! Next time you smell a cake making, stop 10______ and pay attention to your mood (心情). |