• 2022-06-19
    一个有效的工具变量应满足如下两个条件:
    A: ‍[img=238x25]1803831b3d6eca3.png[/img]
    B: [img=248x25]1803831b4757fd4.png[/img]
    C: [img=248x25]1803831b5229971.png[/img]
    D: [img=248x25]1803831b5d14961.png[/img]