请从H、C、O、K中选择合适的元素，按下列要求写出化学式（各写一例）：
①酸性氧化物______；②由三种元素组成的酸______；
③可溶于水的碱______；④由三种元素组成的盐______．
（1）二氧化碳能和碱反应生成盐和水，属于酸性氧化物，酸性氧化物一般是非金属氧化物．故填：CO2．
（2）碳酸是由氢元素、碳元素和氧元素三种元素组成的酸．故填：H2CO3．
（3）氢氧化钾属于碱，易溶于水．故填：KOH．
（4）碳酸钾是由钾元素、碳元素和氧元素组成的盐．故填：K2CO3．
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| Many animals use some kind of "language " They use singals that have meanings. For example, when a bee has found some food, it goes 1 its home. It’s 2 for a bee to tell 3 bees where the food is by speaking to 4 , but it can do a little dancing. This tells the bees where the food is and 5 it is. Some animals show how they feel by making sounds. It’s not difficult to tell if a dog is angry 6 it barks.Birds made several different sounds and 7 has its own meaning. Sometimes we human make sounds 8 we make sounds like“Oh!” or “Ouch!” to show how we feel about something or when we 9 something on our feet. We humans have languages. We have words. These words have the meaning of things. actions, feelings or ideas. We 10 give each other information. By writing down words, we can remember what has happened or send messages to people far away. | |||
| ( )1. A. out of ( )2. A. important ( )3. A. each other ( )4. A. it ( )5. A. how long ( )6. A. so that ( )7. A. each ( )8. A. in the same sound ( )9. A. put ( )10. A. able to | B. on with B. difficult B. another B. him B. how far away B. before B. every B. by the different sound B. drop B. are able to | C. away from C. necessary C. the other C. them C. how many C. until C. all C. in the same way C. fall C. are able | D. back to D. impossible D. others D. themselves D. how old D. because D. some D. by the different way D. set D. be able to |