The small town is so good a place _____ many people come to visit ____  it has become well known all over the world.

A．that ; as

B．as ; as

C．that ; that

D．as ; that

如果阳床进水的硬度=2mmol/L，含钠离子=1mmol/L，经阳离子交换后生成的氢离子应有多少？