A:源远流长
B:博大精深
C:兼收并蓄
D:求同存异
如图,已知 AF⊥平面ABCD,四边形ABEF为矩形,四边形ABCD为直角梯形,∠DAB=90°,AB∥CD,AD=AF=CD=2,AB=4.
(I)求证:AC⊥平面BCE;
(II)求三棱锥E﹣BCF的体积.
He ________ to escape from the prison,but were stopped by the guards.( )
下列语句中,标点符号使用不正确的一项是( )