阅读下列材料，运用所学知识，解决问题
材料一：陕变事起，曾……通电主张一致对外……不图讨伐令忽然而下，值兹强邻压境，国家民族，危在旦夕，方谋解救之不暇，何忍再为箕豆之煎，况汉卿（即张学良，字汉卿）通电各项主张，多位国人所同情者，屡陈不纳，迫以兵谏。绝不宜以叛逆目之。而政府逮加讨伐，宁不顾国人责以勇于对内，怯于对外？”
──1936年12月24日《桂林日报》
材料二：“大义当前，不容反顾，只求于救亡主张贯彻，有济于国家，为功为罪，一听国人之处置。”
──张学良、杨虎城通电全国的电文
请回答：

白天我们能从不同方向看到桌子等物体，这是因为（　　）

《梦溪笔谈》记载：“皇佑（宋仁宗年号）二年，吴中人饥，殍瑾枕路。是时范文正（范仲淹，字希文）领浙西发粟及募民存饷（粮食），为术甚备……义召诸佛寺主首，谕之曰：‘饥岁工价至贱，可以人兴十木之役。’丁是诸寺工作鼎兴。”范仲淹的救灾措施的主要特点是

    I'm Sally . I was a shy girl . Bu this term , I decided to do a brave（勇敢）thing . In this term , we had a new subject to learn –physics . When our teacher asked someone to be the physics monitor（物理课代表）, I put up my hand and got the role . But it was not as easy as I thought .

    At the beginning , I found that many students did not hand in（上交）their homework , so I was a little worried . I shouted at the students . It made the students angry . One girl asked : “ Why are you shouting at me ? You're making it seem like it is all my mistake（错误）. ” Then I found that , it was my mistake in fact . I should talk to my classmates in a kind way and not get so angry .

    After this , I changed（改变）how I spoke . “ Excuse me . Could you please finish your homework ? ” I said . Later , I was surprised that many students understood me a lot better and finished their homework quickly.

    I found the best way to do this job is to talk with my classmates in a friendly way . Now , I am getting better and better with my classmates . This job makes me feel like I am growing up like a little tree day by day .

圆可以分割成 ____个扇形，每个扇形都是由 ____．