已知四边形ABCD的对角线AC、BD相交于点O，下列条件中能够判断有一组对边平行的是（　　）

A．AD：BC=AO：CO

B．AD：BC=DO：CO

C．AO：BO=CO：DO

D．AO：BO=DO：CO

No one told us          ,so we need your help.[ ]

A. how should we do  

B. what should we do        

C. how to do it          

D. what to do it