用于建筑保温系统的保温材料根据燃烧性能等级的不同，主要分为（）级。

A.A

B.B1

C.B2

D.B3

E.B4

People at the party worried about him, because no one was aware（）he had gone.

A.of where

B.of the place where

C.where

D.the place