按先后顺序排列, PPPoE的discovery所包含的4个阶段为( )
APADO->PADI->PADR->PADS
BPADO->PADI->PADS->PADR
CPADI->PADO->PADR->PADS
D PADI->PADO->PADS->PADR
APADO->PADI->PADR->PADS
BPADO->PADI->PADS->PADR
CPADI->PADO->PADR->PADS
D PADI->PADO->PADS->PADR