某口袋中装有红色、黄色、蓝色三种颜色的小球（小球出颜色外完全相同）共60个．通过多次摸球实验后，发现摸到红球、黄球的频率分别是30%和45%，由此估计口袋中蓝球的数目约为____个．

如图所示的几何体是由一些正方体组合而成的立体图形，则这个几何体的俯视图是（　　）

南极地区，是人类最后到达的大陆，被称为“第七大陆”，关于南极你了解多少呢？完成下题．

北京航天飞行控制中心对“嫦娥二号”卫星实施了降轨控制，卫星成功由轨道半径为a，周期为T₁的极月圆轨道进入远月点距离为a，周期为T₂的椭圆轨道，为在月球虹湾区拍摄图象做好准备，轨道如图所示．则“嫦娥二号”（   ）

假定英语课上老师要求同桌之间交换修改作文，请你修改你同桌写的以下作文。文中共有10处语言错误，每句中最多有两处。每处错误仅涉及一个单词的增加、删除或修改。
增加：在缺词处加一个漏字符号（∧）并在其下面写出该加的词。
删除：把多余的词用斜线（\）划掉。
修改：在错的词下划一横线，并在该词下面写出修改后的词。
注意：1）每处错误及其修改均仅限一词；
2）只允许修改10处，多者（从第11处起）不计分。
    Last summer I go to America and studied at a language school. I had many wonderful experience, but I also had a sad one. One day, the school held party , where I invited to talk about Chongqing. After that they asked me a lot of things about China. But I couldn't explain them with English clearly. I felt sadly. I learned a lesson from this experience. I have already studied English for eight years, I can't use it very good. I must work hard to improve my spoken English so that I will not be able to communicate freely with foreigners. I hope I can be a bridge between China and others countries in the future.