P(AB-)=P(A)-P(AB)即0.3=0.5-P(AB)P(AB)=0.2——A对P(A)P(B)=0.5×0.4=0.2=P(AB)故A，B相互独立——C对P(A-|B)=P(A-B)/P(B)=[P(B)-P(AB)]/P(B)=(0.4-0.2)/0.4=0.5——B对选D
