地球的陆地面积占全球面积的百分比是( )
A:21%
B:71%
C:29%
D:79%
(Ⅰ)求证:平面PBC⊥平面PMA;
(Ⅱ)求四面体M﹣AND的体积.
Please _____what your teachers talk about in class, it is important to you.