在Na₂SO₄和Al₂（SO₄）₃的混合溶液中，测得Al³⁺浓度为0.1mol/L，SO₄^2-浓度为0.3mol/L，则混合溶液中Na⁺的浓度为（　　）

A．0.15mol/L

B．0.45mol/L

C．0.6mol/L

D．0.3mol/L

选择正确的引导词补全句子

1.I don't know ________ he will come.(that,if)

2.I believe ________ he is a good teacher.(that,if)

3.I want to know ________ you join this club.(why,what)

4.They asked ________ I came from.(if,where)

5.I can't remember ________ I bought this bag.(when,that)