原水经Na型离子交换器处理后，水中碱度（）
A、增大
B、缩小
C、不变
参考答案：C
My father _________ Beijing on business for three days. He will be back in two days.
A．have gone to
B．has gone to
C．has been to
D．has been in
煤气管道接地电阻不应大于（）欧姆.
A、15
B、10
C、5
