青少年应幸关心社会的兴趣和情感，养成亲社会行为，乐乐的以下行为属于亲社会的是（    ）

①乐乐主动参加了“欢送抗洪英雄”队伍   ②为创建文明社区，乐乐光荣地成为一名“社区文明创建”宣传员

③乐乐在爸爸的指导下学会了游泳         ④乐乐在老师的带领下参观了当地的名胜古迹，学到了很多知识

已知数列{a_n}是等差数列，且a₁+a₅=12，a₄=7，则a_n=____（n∈N^*）．

下列物质的用途主要不是由其化学性质决定的是  （  ）

如图是人们常见的几种动物，请分析回答：

阅读下文，回答问题
                                                                                                  On the beach
    It is a fine day. A lot of people go to the beach to enjoy sunshine. Some people are swimming like fishes in the sea. On the beach，some young people are playing volleyball.
    Some of them are lying under the coconut trees. And some old people are chatting with each other. What are the children doing? Look! They are making castles with spade and sand. They are having fun。
⑴⑵⑶⑷⑸根据短文内容，判断正误；⑹根据短文内容填空。