图中①、②两条曲线为我国某河流修建水库前后的流量年变化曲线，据此判断下列叙述正确的是（　　）

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：其中正确的结论的个数为（　　）

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．

注意：行文连贯，涵盖要点，适当发挥。词数100左右。
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