圆周长C与圆的半径r之间的关系为C=2πr，其中变量是____ ，常量是____ ．

    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.

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