按先后顺序排列， PPPoE的discovery所包含的4个阶段为：________。
A.A.PADO->PADI->PADR->PADS
B.B.PADI->PADO->PADS->PADR
C.C.PADI->PADO->PADR->PADS
D.D.PADO->PADI->PADS->PADR
正确答案是C