In America，primary school lasts six years. There are about twenty students in one class. They are in different groups. Different groups may have different textbooks. The whole grade graduate(毕业)at the same time，but the students learn different knowledge.

    Primary school students don't have schoolbags. Their books are in the school. The students usually study reading，arithmetic(算术)，language，science，music，art and PE. The performances(成绩)show three grades. G means good. S means satisfaction. N means you need to work hard.

“从某种意义上讲，对手是我不可缺少的朋友。感谢对手，因为他们，我才更深刻地看到自身的不足。”晓伟的这一反思显示出他对竞争作用的理解是（    ）

右图为某草原生态系统食物网，请据图回答问题：

（1）这个食物网中有____ 条食物链。
（2）如果青蛙突然减少，蛇的数量将会发生怎样的变化？____ 。（填增多或减少）
（3）如果这个生态系统中的草受到了有毒化学物质的污染，那么这种有毒物质在哪种动物体内积累得最多____ 。
（4）此图说明了各种生物之间的关系是____ 与____ 的关系。