A是正确选项
A是“如上所诉”的固定搭配,但不像1搂说的,as绝对不是做主语.该句子的主语是the atmosphere.
B选项严格来讲在语法上是正确的.it作为形式主语,后面的the atmosphere 作为逻辑主语.但是相比较而言显得拖沓.况且As was mentioned above是习惯用法,约定俗成.
C.what was mentioned above后面应该加was that,然后去掉逗号.否则句子就缺少谓语.后面的 consists of只是从句中的谓语.
D.it was mentioned above that.