根据短文理解，从A、B、C、D四个选项中选择最佳答案填空。

    Xiao Lin has a good friend. Her 1 is Lucy. She is from the USA. She's a(n) 2. Xiao Lin and Lucy  3 in the same class. They go to school five days a4. They stay at  5 on Saturday and Sunday. Lucy 6 China and Chinese food. Her favorite  7 is rice cake. At school they play football after class. Xiao Lin and Lucy like 8 things. Now they are making a plane. They like flying planes on Saturday and Sunday. Lucy 9 English and a little Chinese. They are  10 friends.

下列哪种血管的内表面有防止血液倒流的瓣膜（    ）

设$x,y,z$是不相等的三个数，则使$x,y,z$成等差数列, 且$x,z,y$成等比数列的条件是（   ）

预测一个国家或地区人口数量未来动态的信息主要来自于（　　）