it的指代问题 语法

cduserforgmat

It 在语法题里面，总是有容易错的选项。而且觉得很难判断。
PREP上的一道题： The decline of the mountain yellow-legged frog in the high reaches of the Sierra Nevada has
become so severe that the United States Fish and Wildlife Service could well list it。
因为it是精确的指代修饰的结构，所以我认为 it 指代的是 The decline of the mountain yellow-legged frog 可是觉得不合逻辑了。

讲解说的是 it 指代 的是 只是the mountain yellow-legged frog 。 可是the decline of ...frog不是一个整体被指代吗？ 搞不懂了
大家帮帮忙 发表一下一件！谢过！

cduserforgmat

还有一个：
觉的应该选D啊。 伴随结果么！如果是E： and + so 不是都是连词吗？
Unlike frogs that metamorphose from tadpoles into adults within a one-year period, --------------。
(D) mountain yellow-legged frogs of the Sierra Nevada take three to four years until they reach
adulthood, thus
(E) mountain yellow-legged frogs of the Sierra Nevada take three to four years to reach adulthood,
and so they are
有谁可以给解释一下呗？

cduserforgmat

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cduserforgmat

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