Hi, I am a Chinese boy (中国男孩). My name is Chen Yang. My first name is Yang. My family name is Chen. My English name is Dale. I like (喜欢) red. My telephone number is 178­4267.I have a good friend. He is English (英国人)．His name is Jim Smith. Jim is his first name and Smith is his last name. His Chinese name is Huang Qiang. His phone number is 362­4597.

B

    I'm Sara. I'm a middle school student. At school, we have nine subjects — math, English, Chinese, science, geography, history, art, P.E. and music. My mother is a Chinese teacher and she says Chinese is beautiful and important (重要的). I like Chinese. I always read Chinese books before I go to bed. I also like history. It is my favorite subject. Mr. Brown is great fun. In his history class, I can learn many interesting stories.

    Mr. Green is my math teacher. He says math is useful and interesting, but I think it is boring. My science teacher, Mrs. Smith, is nice. I listen to (听) her carefully (认真的) in class. But I can't do well in the science exam(考试). I don't know why. This subject is really difficult for me.

    We always have art, P.E. or music in the afternoon. For me, the classes in the afternoon are fun and relaxing. In P.E. class, I like to play ping-pong with my P.E. teacher Mrs Miller plays very well.

根据材料内容简要回答下列问题

①碱金属单质的熔点随原子序数的增大而降低

②砹（At）是第VIIA族，其氢化物的稳定性大于HCl

③硒（Se）的最高价氧化物对应水化物的酸性比硫酸弱

④第二周期非金属元素的气态氢化物溶于水后，水溶液均为酸性

⑤铊（Tl）与铝同主族，其单质既能与盐酸反应，又能与氢氧化钠溶液反应

⑥第三周期金属元素的最高价氧化物对应水化物，其碱性随原子序数的增大而减弱．

如图，在△ABC中，∠C=90°．

（1）用尺规作图法作AB边上的垂直平分线DE，交AC于点D，交AB于点E．（保留作图痕迹，不要求写作法和证明）；

（2）连结BD，若BD平分∠CBA，求∠A的度数．