棱长为1的正方体ABCD A₁B₁C₁D₁中，点M、N分别在线段AB₁，BC₁上，且AM=BN，给出以下结论:
①AA₁⊥MN
②异面直线AB₁，BC₁所成的角为60°
③四面体B₁ D₁CA的体积为$\frac{1}{3}$
④A₁C⊥AB₁，A₁C⊥BC₁,，其中正确的结论的个数为(　　)

判断题：在生态系统中，通常每个物种的数量经常处在一个动态变化之中。

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