—Catherine, I have cleaned the room for you.
— Thanks. You _______ it. I could manage it myself.
A:needn't have done
B:needn't do
C:mustn't do
D:shouldn't have done
有空心的铜球、铁球、铅球各一个,体积和质量都相等,将其空心部分都装满水后,质量最大的球是( )
如图,直线l1,l2交于点A,观察图象,点A的坐标可以看作方程组____的解.
①尊重各民族的语言文字
②实行民族区域自治制度
③帮助少数民族地区发展经济
④尊重各民族的宗教信仰、风俗习惯