因为质量相同,则他们对容器最底部的一点的压强是相等的.设水深为h1,盐水深为h2,则ρ水gh1=ρ盐gh2h2=ρ水h1/ρ盐
设AB处的点MN离容器最底部的距离为h3,
则PM=ρ水g(h1-h3)=ρ水gh1-ρ水gh3
PN=ρ盐g(h2-h3)=ρ盐gh2-ρ盐gh3
将ρ水gh1=ρ盐gh2代入,得
PN=ρ水gh1-ρ盐gh3
∴PM-PN=ρ水gh1-ρ水gh3-(ρ水gh1-ρ盐gh3)
=ρ盐gh3-ρ水gh3=(ρ盐-ρ水)gh3
∵ρ盐>ρ水∴PM-PN>0即PM>PN.选A.