某四棱台的三视图如图所示,则该四棱台的体积是 ( )
(Ⅰ)求证:MN∥平面ACC1A1;
(Ⅱ)求证:MN⊥平面A1BC;
(Ⅲ)求三棱锥B﹣A1NC的体积.
(I)求证:GH⊥DM;
(II)当三棱锥D-MGH的体积最大时,求点A到面MGH的距离.