书面表达。

    美国学生Sam 最近又来到了北京，去了王府井，发现这里发生了巨大的变化。请你以Sam身份给他

父母写一封信，介绍王府井发生的变化。字数80左右，信的开头及结尾已给出。

     提示词：1. happy, visit, again 2. change, take place 3. Wangfujing Street, building, shop, market

                   4. flower, poster (宣传画), put, for, the Olympics

Dear Mum and Dad, 

                                                                                                                                                 Yours,

                                                                                                                                                 Sam

随机对照临床试验结果属于（）

A.一级证据

B.二级证据

C.三级证据

D.四级证据

E.五级证据