A:to blame; occurred
B:to blame; occurring
C:to be blamed; occurred
D:to be blamed; occurring
(Ⅰ)求证:PO⊥平面ABCD;
(Ⅱ)求证:OE∥平面PDC;
(Ⅲ)求面PAD与面PBC所成角的大小.
下列关于生物进化总趋势的叙述,不合理的是( )