A:may
B:should
C:must
D:shall
(Ⅰ)当平面PBD⊥平面PBC时,求三棱锥P﹣BCD的体积;
(Ⅱ)在图2中,E为PC的中点,若线段BQ∥CD,且EQ∥平面PBD,求线段BQ的长;
(Ⅲ)求证:BD⊥PC.
说出下列消防安全标志的含义
A____
B____
C____
— _______________( )
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