—Jack w____a whole afternoon in playing computer games．

—That's too bad．He should spend more time studying．

If you want people to treat you nicely, you have to treat        nicely, too.                    

某同学利用天平和量筒测量小石块的密度，下列操作步骤中不必要的是（　　）

下列各对性状中，属于相对性状的是（  ）