A:the more. the better
B:easier said than done
C:better than never
D:better than nothing
①当点N是棱B1C1的中点时,MN∥平面ACC1A1;
②MN⊥A1C;
③三棱锥N﹣A1BC的体积为VN﹣A 1 BC= 16 a3;
④点M是该多面体外接球的球心.
其中正确的是____.
下列物质中属于纯净物的是( )