党的基本路线可以概括为“一个中心，两个基本点”，下列实践中能直接体现“一个中心”的是 （   ）

雨后，常见地面上有蚯蚓，是因为（  ）

阅读理解
B
    Hello!I'm Kate．I want to talk about my smart phone(智能手机)．My smart phone is white．It's from my uncle Sam．He works in New York and comes back to London every December．I like mv smart phone．It is useful.
    The school rules don't allow(允许)us to take smart phones to school,so I usually use it after school or on the weekend．When I go out with my friends，I like keeping it in my bag．So my family can call me at any time．When I don't go to school．I like to call my friends to talk to them，or ask them for help with my study．I'm not very good at math，so I put an APP(应用软件)about math in my smart phone to help me． It is fun and helpful．I also use the smart phone to do some reading，play games，and watch movies．Isn't it great?
    Sometimes smart phones are not good for people．Some people spend too much time on it．I like my smart phone and I know how to let it help me. It's my good friend．
根据材料内容选择最佳答案。

根据史书记载，西周初年的国家有800多个，到了春秋初年，还剩下170多个，战国之初，只有十几个了。这反映了历史发展的趋势是（    ）

将具有1对等位基因的杂合体，逐代自交3次，在F₃代中纯合体比例为（    ）