∴MB∥ aircraft DNC

∵AMND is a rectangle,

∴MA∥DN.

Horses again? Aircraft DNC, DN? Plane DNC,

∴MA∥ aircraft DNC ..

And MA∩MB=M, and MA, MB? Airplane AMB,

∴ plane AMB∑ plane DNC ..

(ii)∫amnd is rectangular,

∴AM⊥MN.

∫ plane AMND⊥ plane MBCN, and plane AMND∩ plane MBCN=MN,

∴AM⊥ Aircraft Company.

BC? Aircraft MBCN,

∴AM⊥BC.

∵MC⊥BC,MC∩AM=M，

BC⊥ Aircraft AMC ..

∵AC？ Aircraft AMC,

∴BC⊥AC.