设一平面图G有 n个顶点, m 条边, r 个面，k个连通分支，则下列哪个是欧拉公式：

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【单选题】【消耗次数：1】

① m - n + r = 2；

② n - m + r = k + 1；

③ n - r + m = k + 1；

④ n - m + r = 2。

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