将两枚铁钉分别放在空气中和潮湿的纸巾上，则生锈较快的是放在____的铁钉。

    Most animals protect(保护) their young until they are old enough to take care of themselves. A fish that swims in the rivers of South America protects its young in a very strange way. This fish, a kind of catfish, uses its mouth to keep its babies from danger(危险).
    When this catfish lays(产卵) its eggs, it carries them in its mouth. From what we know, it does not eat during this time. After the eggs hatch(孵化), the fish swims with its young to protect them. It opens its mouth when danger appears(出现), and the tiny fish swim inside. They swim out only when it's safe again.
    These catfish may have a peculiar way to protect their young, but it seems(好像) to work.

探究下列材料，写出你的发现。
       材料1：爷爷一提到“粮票”“烟票”“人民公社”就感慨不已。对他的孙女说，这些词语太陌生了，她经常说的是“上网”“粉丝”“低碳”等新词语。
       材料2：某个娱乐节目搞“海选”，不久到处都“海选”；电视台刚播《蜗居》，老百姓全在说“蜗居”。这些名称一出现，就成了各大媒体报道中的热闹词汇。
       材料3：李老师在批改随笔时，看到了天书般的一段话：“偶灰常稀饭周杰伦，那天偶拿他的片片到班里亮骚，被老斑发现收了去，真是7456……”李老师当场“晕倒”。
      ①你的发现：
      ②你的观点：

东汉末年，关中地区人口从240万减少到50万，扬州从320万增加到433万，荆州（今湖北境内）从359万增加到620万．导致这一时期南北人口变化的主要因素是（　　）