关系模型的完整性规则是对关系的某种约束条件，包括实体完整性、______和自定义完整性。

根据短文内容和汉语提示，用所给词的正确形式填空。每空限填一词。（10分）

The study of English in Grade Three is really interesting although it is a    小题1:      

(艰苦的) task for most of us to learn it well. Yes, not easy. In the last few weeks, we have learned about the  小题2: (自然的) wonders of the world, the adventures of Tom Sawyer, Liu Xiang and the great  小题3: （发明） of paper and printing. When you stand on the edge of the Grand Canyon, you can feel the ground fall away and down to a river, far    小题4:  （在下面） you. How great it is! Mr Jackson’s favourite   小题5:  （场景） of the book by Mark Twain is that people are  小题6:   （高兴） to see Tom 小题7: （活着的）. Liu Xiang’s story tells us to work hard so that we can  小题8: （打败）the difficulties by ourselves. We also learned that people are thinking about if paper will be  小题9:  （替代）by computers. However, looking 小题10:（浏览）a magazine at hand is easier than turning on a computer. Don’t you think so?