物质种类

waste matter───[环境]废物

conservation of matter───物料守恒

centre of masses───[力]质心；质量中心

class number───[数]类数；[计]分类号；[数]组数

class societies───阶级社会

class teacher───班主任；班级教师

classmate───n.同班同学

But these are merely lifestyle choices for the affluent middle-classes, a matter of aesthetics, and nothing to do with the real issues.───物质变化分为两大类：物理变化和化学变化。