By the time he realizes he ______ into a trap, it'll be too late for him to do anything about it. [      ]

A. walks

B. walked    

C. has walked      

D. had walked

目前下列工艺过程没有直接使用离子交换技术的是

A．硬水的软化

B．电解饱和食盐水制造NaOH

C．电渗析淡化海水

D．海水中提取金属Mg