一个直角三角形有两条边长为3，4，则较小的锐角约为（　　）

阅读理解

    It is Sunday today. It is my grandma's birthday. Father, mother and I go to visit her.

    She lives with my uncle. In the morning we buy a birthday cake and some fruits and go to her house at ten o'clock.

    We spend about half an hour getting there. My grandma and uncle are very glad to see us. We give the presents to grandma and say“Happ y birthday to you! ”She is very happy. Then we sit down to talk. My uncle can cook.

    The meal is very nice. We all like it very much. After lunch, I play with my uncle. At four o'clock, we say goodbye and go home.

材料一  清朝末年，国家内忧外患，人民处于水深火热 之中，而当权者却贪图享乐。为此，有人写了一副对联，上联是：“今日幸南苑，明日幸北海，何日复幸古长安？”下联是：“五旬割琉球 ，六旬割台湾，七旬又割东三省，每逢万寿无疆。”

材料二  云龙石雕是故宫最大的一块石雕，原产于北京房山的大石窝村......从大石窝村到北京不下50多公里，竟用了2万多民工，历时20多 天，花了10多万两白银才运到。

                                                           保和殿 后的云龙石雕

（I）证明：AE⊥CD

（II）在棱ED上是否存在点M，使得直线AM与平面EFBD所成角的正弦值为 $\frac{\sqrt{6}}{3}$ ？若存在，确定点M的位置；若不存在，请说明理由．