己知集合A={1,2,3,4},B={x[x2﹣2x﹣3≤0},则A∩B=( )
A:{1}
B:{1,2}
C:{1,2,3}
D:{1,2,3,4}
(Ⅰ)求证:平面PBC⊥平面PMA;
(Ⅱ)求四面体M﹣AND的体积.
— Did you see Anna after class?
— I saw her leave the classroom, but I didn't notice __________after that.