题干
In the writer's opinion, the most difficult thing for him was __________.
A：to start with elementary things
B：to have faith in himself
C：to fight with blindness
D：to make adjustments to reality
答案(点此获取答案解析）
B
的正方形，则以该空间几何体各个面的中心为顶点的多面体的体积为( )．