阅读下面短文，从A、B、C、D四个选项中选出最佳选项。

    This is a picture of Mr. Li's family. The man in the middle is Mr. Li. The woman is Mrs. Li's wife. They have two sons. The child behind Mr. Li is Li Lei. He's thirteen.  The boy in front of Mrs. Li is Li Ming. He is seven. Li Lei and Li Ming are in the same school, but not in the same grade. Li Lei is in Grade Two. Li Ming is in Grade One. They are good students.

有下列4个命题：

①若函数f（x）定义域为R，则g（x）=f（x）﹣f（﹣x）是奇函数；

②若函数f（x）是定义在R上的奇函数，∀x∈R，f（x）+f（2﹣x）=0，则f（x图象关于x=1对称；

③已知x₁和x₂是函数定义域内的两个值（x₁＜x₂），若f（x₁）＞f（x₂），则f（x）在定义域内单调递减；

④若f（x）是定义在R上的奇函数，f（x+2）也是奇函数，则f（x）是以4为周期的周期函数．

其中，正确命题是____ （把所有正确结论的序号都填上）．

— I have ______ completed all the work.

— Congratulations!