设是虚数单位，则 是（   ）     

       One evening , my dad asked me to buy some bread for dinner . It was dark and cold outside . I rode a bike to a shop near my school . When I left the shop ,it got even darker , so I got on my bike right away. Then I found a woman in a white dress riding a motorcycle after me . She followed me for a long time . I rode very fast and started to cry for help, but no one was there . I was too scared and too tired to ride any faster . At last , I gave up . The woman stopped in front of me and said , “ Why were you riding so fast , Ken ?It’s dangerous !” I looked at the woman . “ Oh , It's you, mum !You really scared me . Dad said you wouldn't be back for dinner tonight !”

想一想，填一填

○○□□□○○□□□○○□□□……

第30个图形是____．

下列物质中，以自由扩散方式出入细胞速率最快的是（    ）

统计

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