在NaCl、MgCl₂和MgSO₄三种盐配成的混合溶液中，若Na⁺的浓度为0.1mol/L、Mg²⁺的浓度为0.25mol/L、Cl^-的浓度为0.2mol/L，则SO₄^2-的物质的量浓度为（　　）

A．0.5mol/L

B．0.45mol/L

C．0.2mol/L

D．0.25mol/L

We are taught that a business letter should be written in a formal style __ in a personal one.

A．rather than

B．better than

C．more than

D．less than