如图，小敏同学想测量一棵大树的高度．她站在B处仰望树顶，测得仰角为30°，再往大树的方向前进4m，测得仰角为60°，已知小敏同学身高（AB）为1.6m，则这棵树的高度为（   ）（结果精确到0.1m， $\sqrt{3}$ ≈1.73）．

4的算术平方根是（）

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