产品数据管理(PDM)软件分为非嵌入式及嵌入式两种。( )
参考答案：对
It is the first time I ______ this kind of music.
A．heard
B．am hearing
C．have heard
D．had heard
齿面硬度＞350HBS的齿轮称为______齿轮。
