某县自来水收费是这样规定的：每户每月用水在12吨以内（含12吨）按0.8元一吨收费，超过12吨的，其超出部分按2.8元收费．某户五月份用水25吨，应交多少元水费？

The real-name policy for train tickets has been applied to all trains since January 1,2012 in China, ______ I think will make it easy for people to get tickets.

A．which

B．when

C．what

D．where