东北地区北部分布着肥沃的（　　）

我国照明用的交流电的电压是____V，频率是____Hz．

已知tanα，tanβ是方程2x²+3x﹣7=0的两个实根．

下列几种物质都是中学化学常见的酸，在相同条件下，酸性最强的是（    ）

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。