A:文献记载
B:口头传说
C:考古发现
D:天象观测
计算: 2ba·a24b2c =____
—Are you sure this is a photo , the famous comedy actress?
—It surprised you, didn't it? But she was once really thin.
天坛公园里的回音壁是我国建筑史上的一大奇迹,回音壁应用的声学原理有( )①声音的反射 ②声音在空气中的传播 ③声音在墙壁内的传播 ④利用回声增强原声的现象