有X、Y、Z三种金属，如果把X和Y分别放入稀硫酸中，X溶解并产生氢气，Y不反应；如果把Y和Z分别放入硝酸银溶液中，过一会儿，在Y表面有银析出，而Z没有变化。根据以上实验事实，判断这三种金属的活动性由强到弱的顺序为（）

      Lily has a pen friend in America. Her name is Alice. They write to each other every week. They tell each other about their lives. Here is a letter from Lily.

Dear Alice,

    Thank you for your letter and telling me about your best friend. In this letter, I’m going to tell you about my friend. Her name is Cathy. She has long brown hair and blue eyes. She is the same age as me, but a little taller .She is always happy. We sit together in class and help each other with our homework. We always play together after school and play computer games for an hour. Cathy thinks quickly（迅速地） and does better than me. She is learning to play the piano（钢琴）.I am learning to play the piano, too. We play together every day. It’s fun. We always have a good time.

    Please write to me soon.

                                                                                                                                                                                                   Love,

                                                                                                                                                                                                     Lily

如图，Rt△ABC中，∠C=90°，AB=10，AC=8，E是AC上一点，AE=5，ED⊥AB于D．

（1）求证：△ACB∽△ADE；

（2）求AD的长度．