公元前221年的某一天,嬴政在宫中大宴群臣,举国一片欢腾,他们最有可能是在庆祝( )
A:灭六国,建秦朝
B:商鞅变法成功
C:灵渠的开通
D:长城的修筑
Although you are young, you can ____ (仍然) make the world more beautifulin your own way.
(Ⅰ)求证:PE⊥BC;
(Ⅱ)求证:平面PAB⊥平面PCD;
(Ⅲ)求证:EF∥平面PCD.