阅读下文，完成（1）～（5）题。

木 兰 歌

                                                                                                                           【唐】韦元甫

        木兰抱杼嗟，借问复为谁。欲闻所戚戚，感激强起颜。老父隶兵籍，气力日衰耗。岂足万里行，有子复尚少。胡沙没马足，朔风裂人肤。老父旧羸病，何以强自扶？木兰代父去，秣马备戎行。易却纨绮裳，洗却铅粉妆。驰马赴军幕，慷慨携干将。朝屯雪山下，暮宿青海旁。夜袭燕支虏，更携于阗羌。将军得胜归，士卒还故乡。父母见木兰，喜极成悲伤。木兰能承父母颜，却卸巾帼理丝黄。昔为烈士雄，今复娇子容。亲戚持酒贺，父母始知生女与男同。门前旧军都，十年共崎岖。本结兄弟交，死战誓不渝。今者见木兰，言声虽是颜貌殊。 惊愕不敢前，叹重徒嘻吁。世有臣子心，能如木兰节，忠孝两不渝，千古之名焉可灭！

完形填空，阅读短文，根据短文选出最佳答案。

                                                                                       Shopping on Taobao

    Where do you often go shopping?

  “Who needs a shopping mall 1we have Taobao?”2 Wang Lin. she is 28 3 old . She is a writer in Beijing. Taobao is China's largest online shopping website. Shopping on Taobao is an important part of Wang Lin's life. She spends lots of money 4Taobao every day.

    Now 5 people like Wang Lin find the joy of shopping online. Among(在……之中) them, women are much more than 6, because they can seldom say beautiful clothes.

   “Taobao” means “look for treasure”in 7. People can find 8 they need on Taobao, 9 candies to DVD players.

Wang Lin says, “It is very cheap". If you don't like the goods (商品), you can get your 10 back. I like shopping on Taobao.

如图，已知⊙O的半径为4，CD是⊙O的直径，AC为⊙O的弦，B为CD延长线上的一点，∠ABC=30°，且AB=AC．

（1）求证：AB为⊙O的切线；
（2）求弦AC的长；
（3）求图中阴影部分的面积．