It is said that the early European playing-cards ______ for entertainment and education.[ ]

A. were designed

B. have designed  

C. have been designed  

D. were being designed

已知0.1mol/L的二元酸H₂A溶液的pH=4，则下列说法中不正确的是（　　）

A．在NaHA溶液中一定有：c（Na⁺）+c（H⁺）=c（HA^-）+c（OH^-）+2c（A^2-）

B．在Na₂A溶液中一定有：c（OH^-）=c（H⁺）+c（HA^-）+2c（H₂A）

C．在Na₂A和NaHA的两种溶液中离子种类相同

D．在溶质物质的量浓度相等的Na₂A和NaHA溶液中，阴离子总数相等