肾脏的结构和功能的基本单位是                               （    ）

如果一个种群的密度愈来愈小，最可能的原因是

I am a middle-school student. Now let 1 tell you something 2 our classroom. It's very 3 There are two maps4 the back wall. 5is a map of China.6 is a map of the world (世界).There are7 desks and chairs in our classroom. There is a big desk 8 theteacher in the front of it. We 9 four classes in the 10 and two in the afternoon

“北宋时，有人在黏土制成的一个个小方块上刻单字，再用火烧成陶字．排版时，把陶字放在一个铁框里，排满为一版；印刷时可以同时排版，效率很高．印完一版以后，陶字拆下还可以再用。”这里介绍的是（    ）