选择题:I think____ is important for us to have breakfast.

正常应该写成:I think that it is important for us to have breakfast.
但通常引导宾语从句的that可以省略.因此可以写成:
I think it is important for us to have breakfast.

B.it.习惯就是这样了。

B。宾语从句中，it作形式主语。