Jack’s mother kept telling him that he should work harder，but________didn’t help.[ ]

A．he  

B．which

C．it  

D．she

杯中原有浓度为18%的盐水溶液100毫升，重复以下操作2次，加入100毫升水，充分融合后，倒出100毫升溶液，问杯中盐水溶液的浓度变成了多少( )

A．9%

B．7.5%

C．4.5%

D．3.6%